When ingesting messages into a JetStream stream, the speed becomes limited if headers are set on a message due to needing to do all header checks. These header checks currently continuously re-scan the headers for different keys, instead of indexing these headers once and then being able to fetch the values quickly.

This PR proposes an improvement by indexing the headers once and using that throughout for stream ingest. Using an async publish benchmark with the only header ofk: v (just to contain headers and require the server to scan even if the server doesn't need to act on them), for both R1 and R3 there's a performance improvement between 12-26%. Using a R1 withPersistMode: async (async flush) even shows a performance improvement of ~39% (915k msgs/s -> 1.3M msgs/s).

The current implementation of a pointer tojsHdrIndex, the usage ofsync.Pool and referencing[]byte was benchmarked and came out as the most performant when compared with amap to a range or[]byte, without returning a pointer, and when returning a range intohdr instead of[]byte.

Signed-off-by: Maurice van Veengithub@mauricevanveen.com