[FIXED] Hold JS lock when accessing stream assignment

[MauriceVanVeen]

Fixes this data race:

WARNING: DATA RACE
Read at 0x00c0034a4720 by goroutine 19950:
  runtime.mapiterinit()
      /home/travis/sdk/go1.23.3/src/runtime/map.go:877 +0x0
  github.com/nats-io/nats-server/v2/server.TestJetStreamClusterMaxConsumersMultipleConcurrentRequests()
      /home/travis/build/nats-io/nats-server/server/jetstream_cluster_2_test.go:2080 +0xcc6
  testing.tRunner()
      /home/travis/sdk/go1.23.3/src/testing/testing.go:1690 +0x226
  testing.(*T).Run.gowrap1()
      /home/travis/sdk/go1.23.3/src/testing/testing.go:1743 +0x44
Previous write at 0x00c0034a4720 by goroutine 20101:
  runtime.mapassign_faststr()
      /home/travis/sdk/go1.23.3/src/runtime/map_faststr.go:223 +0x0
  github.com/nats-io/nats-server/v2/server.(*jetStream).processConsumerAssignment()
      /home/travis/build/nats-io/nats-server/server/jetstream_cluster.go:4281 +0x834
  github.com/nats-io/nats-server/v2/server.(*jetStream).applyMetaEntries()
      /home/travis/build/nats-io/nats-server/server/jetstream_cluster.go:2015 +0x78a
  github.com/nats-io/nats-server/v2/server.(*jetStream).monitorCluster()
      /home/travis/build/nats-io/nats-server/server/jetstream_cluster.go:1413 +0x1194
  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/build/nats-io/nats-server/server/server.go:3885 +0x59

js.streamAssignment mentions the lock should be held. Also found some other places that didn't lock.

// Will lookup a stream assignment.
// Lock should be held.
func (js *jetStream) streamAssignment(account, stream string) (sa *streamAssignment) {

Signed-off-by: Maurice van Veen github@mauricevanveen.com

[derekcollison]

LGTM