Correctly track lowest closed instant/snapshot seqno

[marvin-j97]

Currently, MVCC versions of items may be GC'd if there is no open snapshot and the level is deep enough.
However, if there's always some snapshot hanging around, it may completely prevent GC of old items.
It would be better to keep track of open snapshots, and store the lowest seqno that can be definitely be dropped without affecting existing snapshots, and using that number inside compaction filtering.

Example:

LowestSeqno = 0 // means nothing can be cleaned up

OpenSnapshot(1)
OpenSnapshot(2)

CloseSnapshot(2)
LowestSeqno = 0 // because Snapshot=1 is still hanging around, the counter was not changed

CloseSnapshot(1)
LowestSeqno = 2 // means we can remove any value that has seqno < 2

Note: There can be multiple snapshots with the same seqno

Possible API

SnapshotTracker::default() => starts at 0
SnapshotTracker::register(seqno)
SnapshotTracker::release(seqno)
SnapshotTracker::get_lowest() -> Option => gets the highest seqno we can safely free

[marvin-j97]

Needs a bit more testing