Fix for data race when changing retention policy

[wallyqs]

Should fix following data race:

WARNING: DATA RACE
Write at 0x00c0006c3cf8 by goroutine 65593:
  github.com/nats-io/nats-server/v2/server.(*stream).updateWithAdvisory()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/stream.go:1907 +0x19c6
  github.com/nats-io/nats-server/v2/server.(*jetStream).processClusterUpdateStream()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/jetstream_cluster.go:3448 +0xde5
  github.com/nats-io/nats-server/v2/server.(*jetStream).processUpdateStreamAssignment()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/jetstream_cluster.go:3344 +0x904
  github.com/nats-io/nats-server/v2/server.(*jetStream).applyMetaEntries()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/jetstream_cluster.go:1949 +0xcd6
  github.com/nats-io/nats-server/v2/server.(*jetStream).monitorCluster()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/jetstream_cluster.go:1344 +0xde4
  github.com/nats-io/nats-server/v2/server.(*jetStream).monitorCluster-fm()
      <autogenerated>:1 +0x33
  github.com/nats-io/nats-server/v2/server.(*Server).startGoRoutine.func1()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/server.go:3609 +0x277
Previous read at 0x00c0006c3cf8 by goroutine 66031:
  github.com/nats-io/nats-server/v2/server.(*consumer).processAckMsg()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/consumer.go:2675 +0x78a
  github.com/nats-io/nats-server/v2/server.(*consumer).processAck()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/consumer.go:1951 +0x2c6
  github.com/nats-io/nats-server/v2/server.(*consumer).processInboundAcks()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/consumer.go:3683 +0x56a
  github.com/nats-io/nats-server/v2/server.(*consumer).setLeader.func4()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/consumer.go:1262 +0x44

[wallyqs]

This test at the moment also triggers another data race when using memory storage:

go test ./... -race --count=10 -p=1 -v -run TestJetStreamLimitsToInterestPolicyWhileAcking/Memory
==================
WARNING: DATA RACE
Read at 0x00c0000a6e38 by goroutine 412:
  github.com/nats-io/nats-server/v2/server.(*memStore).LoadNextMsg()
      /Users/wallyqs/repos/nats-dev/src/github.com/nats-io/nats-server/server/memstore.go:954 +0x644
  github.com/nats-io/nats-server/v2/server.(*consumer).getNextMsg()
      /Users/wallyqs/repos/nats-dev/src/github.com/nats-io/nats-server/server/consumer.go:3413 +0xb22
  github.com/nats-io/nats-server/v2/server.(*consumer).loopAndGatherMsgs()
      /Users/wallyqs/repos/nats-dev/src/github.com/nats-io/nats-server/server/consumer.go:3824 +0x484
  github.com/nats-io/nats-server/v2/server.(*consumer).setLeader.func3()
      /Users/wallyqs/repos/nats-dev/src/github.com/nats-io/nats-server/server/consumer.go:1259 +0x44

Previous write at 0x00c0000a6e38 by goroutine 336:
  github.com/nats-io/nats-server/v2/server.(*memStore).recalculateFirstForSubj()
      /Users/wallyqs/repos/nats-dev/src/github.com/nats-io/nats-server/server/memstore.go:1070 +0x6a5
  github.com/nats-io/nats-server/v2/server.(*memStore).LoadNextMsg()
      /Users/wallyqs/repos/nats-dev/src/github.com/nats-io/nats-server/server/memstore.go:955 +0x653
  github.com/nats-io/nats-server/v2/server.(*consumer).getNextMsg()
      /Users/wallyqs/repos/nats-dev/src/github.com/nats-io/nats-server/server/consumer.go:3413 +0xb22
  github.com/nats-io/nats-server/v2/server.(*consumer).loopAndGatherMsgs()
      /Users/wallyqs/repos/nats-dev/src/github.com/nats-io/nats-server/server/consumer.go:3824 +0x484
  github.com/nats-io/nats-server/v2/server.(*consumer).setLeader.func3()
      /Users/wallyqs/repos/nats-dev/src/github.com/nats-io/nats-server/server/consumer.go:1259 +0x44


WARNING: DATA RACE
Write at 0x00c00da10a88 by goroutine 3587266:
  github.com/nats-io/nats-server/v2/server.(*memStore).recalculateFirstForSubj()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/memstore.go:1069 +0x687
  github.com/nats-io/nats-server/v2/server.(*memStore).LoadNextMsg()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/memstore.go:955 +0x653
  github.com/nats-io/nats-server/v2/server.(*consumer).getNextMsg()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/consumer.go:3413 +0xb22
  github.com/nats-io/nats-server/v2/server.(*consumer).loopAndGatherMsgs()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/consumer.go:3824 +0x484
  github.com/nats-io/nats-server/v2/server.(*consumer).setLeader.func3()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/consumer.go:1259 +0x44
Previous read at 0x00c00da10a88 by goroutine 3587247:
  github.com/nats-io/nats-server/v2/server.(*memStore).LoadNextMsg()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/memstore.go:955 +0x666
  github.com/nats-io/nats-server/v2/server.(*consumer).getNextMsg()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/consumer.go:3413 +0xb22
  github.com/nats-io/nats-server/v2/server.(*consumer).loopAndGatherMsgs()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/consumer.go:3824 +0x484
  github.com/nats-io/nats-server/v2/server.(*consumer).setLeader.func3()
      /home/travis/gopath/src/github.com/nats-io/nats-server/server/consumer.go:1259 +0x44

[neilalexander]

LGTM

[wallyqs]

Made this PR as a follow up for the other race that happens in this test: #4552

[derekcollison]

LGTM