[FIXED] potential message duplication from sources when downgrading b…

…ack from 2.10 (#4454)

- [X] Changes squashed to a single commit (described
[here](http://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html))
 - [ ] Build is green in Travis CI
- [X] You have certified that the contribution is your original work and
that you license the work to the project under the [Apache 2
license](https://github.com/nats-io/nats-server/blob/main/LICENSE)

2.10 adds a couple space separated fields to the sourcing message header
from 2 to 4 but the current 2.9 code is too strict of checking the
number of fields is exactly 2 rather than at least 2

server/stream.go

@@ -2983,7 +2983,7 @@ func streamAndSeq(shdr string) (string, uint64) {
 	}
 	// New version which is stream index name <SPC> sequence
 	fields := strings.Fields(shdr)
-	if len(fields) != 2 {
+	if len(fields) < 2 {
 		return _EMPTY_, 0
 	}
 	return fields[0], uint64(parseAckReplyNum(fields[1]))