Reduce allocations in WebSockets

[neilalexander]

This PR primarily does two things:

1. Updates the buffer pool to allow reclaiming buffers that have been grown beyond their original capacity by slicing them up into capacities we can make use of (reverted for now);
2. Updates the WebSocket code to remove extra copies, instead drawing straight from the compression buffer, which is reset on each call.

With nats bench ... --pub=1 --sub=3 --msgs=100000 --size=1024 over non-TLS WebSockets, we have gone from:

• 174,688 msgs/sec ~ 170.59 MB/sec up to 834,240 msgs/sec ~ 814.69 MB/sec
• 40GB of wsCollapsePtoNB allocations down to 10MB
• 13GB of flate.NewWriter allocations down to 3.63MB

Signed-off-by: Neil Twigg neil@nats.io

[derekcollison]

LGTM