Collections: Default collection MVS wrong in a number of scenarios (post warmup)

The fix added for MB-50413 has a corner-case issue which was discovered whilst extending this area to support CDC on the default collection.

MB-50413 adds some code to maintain what the default collection's max-visible seqno (MVS) is. The MVS is a concept added in mad-hatter to support DCP clients that don't care for sync-writes prepare/abort. The MVS always represents the vbucket's highest commit (which is visible to all DCP streams).

In 7.0 along came collections, now the vbucket's high-seqno etc... may not be the default collection, yet we may have DCP clients which don't support collections. These clients need their own special MVS, which is what MB-50413 adds.

This bug is the following:

• empty vbucket
• flush a single prepare
• warmup and check default collection MVS (added in MB-50413)

The MVS should be 0, but after warmup it incorrectly reports 1. This is due to broken logic in Manifest::setDefaultCollectionMaxVisibleSeqnoFromWarmup which interprets an input of 0 to have special meaning, when it needs different checks.

This bug is required to go into 7.2 to help fix the problems around "modify of default collection". A backport of this could in theory also go back to 7.1.x