[FIXED] JetStream catchup may return/delete too much

[MauriceVanVeen]

JetStream catchup could have a follower delete too much because it could send too large delete ranges.

This was due to LoadNextMsg being used and not checking if the requested last sequence was exceeded. This could happen if LoadNextMsg was used AND last < state.LastSeq AND there's at least one deleted message at sequence last+1.

Given you have a stream with a message at sequence 1 and 100, with a gap in the middle, there were two issues when requesting only to receive what message is stored at sequence 5 (for example):

• A delete range would be sent with sequence 5 and number of deletes of 95, this should be 1 delete.
• If the message at sequence 100 was deleted it would also return the same as above, this should also be only 1 delete.

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

[derekcollison]

LGTM