[FIXED] Clear all pre-acks for seq upon removing message

[MauriceVanVeen]

If a message in a stream was removed/erased/purged the pre-acks would not be cleared for that sequence. This meant that pre-acks could remain and potentially result in a memory leak.

There was also a call to mset.clearAllPreAcks(last) where the write lock was not held. Updated mset.setLastSeq to require a write lock to already be held, so we don't need to keep locking/unlocking multiple times.

Also de-flakes TestNoRaceJetStreamClusterUnbalancedInterestMultipleConsumers due to:

• remaining pre-acks that should have been cleared
• messages being removed asynchronously due to going through proposals (added checkFor)

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

[derekcollison]

LGTM