Optimize consumer messages sequences for multiple subjects

If consumer with multiple subjects encountered a sequnece
of messages from the same subject, it tried to load messages
from other subjects in some cases.
This checks for that scenario and optimizes it by early returning.

Signed-off-by: Tomasz Pietrek <tomasz@nats.io>

server/consumer.go

@@ -3295,6 +3295,11 @@ func (o *consumer) getNextMsg() (*jsPubMsg, uint64, error) {
 					o.updateSkipped(uint64(filter.currentSeq))
 				}
 			}
+
+			// If we're sure that this filter has continuous sequence of messages, skip looking up other filters.
+			if filter.currentSeq == nextSeq && sseq == filter.currentSeq && err != ErrStoreEOF {
+				break
+			}
 		}
 
 	}