Collections: GetAllVBSeqs returns max-visible seqno to collection aware client for a bucket request

Discovered when testing changes for MB-50413 (and loosely related).

In mad-hatter (durability release) KV-engine tracked the max-visible seqno, this is the highest committed item of a vbucket. For clients that use DCP and GetAllVBSeqnos and do not support sync-replication, this value tracks the highest item DCP will send them, i.e. we won't send a prepare/abort. This fixed a number of issues where DCP clients would hang waiting for DCP to send them a seqno that GetAllVBSeqnos told them about.

In cheshire-cat (collections release) KV-engine made lots of changes to DCP to support collections, included in this was a new DCP message called SeqnoAdvacned. A message that DCP uses in-place of items it wouldn't send. This is used to ensure a snapshot end is reached in the case where a snapshot ends on something the client doesn't know about, e.g. another collection or prepare/abort.

A client can do a bucket DCP stream, and see all collections, but still not enable sync-writes, so they don't see prepare/abort. However when they call GetAllVBSeqs for the bucket high-seqno, we still send them the max-visible seqno, when we technically should send them the high-seqno (because they have stated they support collections ... SeqnoAdvance)

Example:

If the bucket stores (and is quiesced)

• seqno:1 committed mutation key1
• seqno:3 abort key2

A client that is enabling collections (yet does not enable sync-writes, i.e. everyone except KV-engine itself)

• GetAllVBSeqs(collection=c1) returns 1
• DCP stream for c1 returns
• DCP snapshot start=1, end=3
• DCP mutation key1 seqno1
• DCP SeqnoAdvance seqno3

the proposed change will mean in this case GetAllVBSeqs returns 3

Note there is also another discrepancy here as well, if for example the data in the bucket is all in collection c1 and the client targets c1 for all operations.

• GetAllVBSeqs returns 3
• DCP returns
• DCP snapshot start=1, end=3
• DCP mutation key1 seqno1
• DCP SeqnoAdvance seqno3

GetAllVBSeqs when targetting a collection is already returning the high-seqno