Improve in-memory virtual map mode to run garbage collection for merged copies

[artemananiev]

This is a follow-up for #15448, a corner case that was not addressed in that feature.

Assume there are many copies (fast copy versions) of a virtual map, and some copy is not released for some reason:

• Since the copy is not released, it cannot be flushed or merged
• The next copy after that can be merged, though, and the next after next, too
• Copies are merged till at some point (some version) a copy is so large that its size exceeds flush threshold, and it can't be merged any longer
• The next copy after that can still be merged
• And so on

So in the end the list of copies in the virtual pipeline will look like this (newest to oldest):

• mutable copy
• immutable copy version N, contains changes from versions N - X + 1 to N
• immutable copy version N - X, contains changes from version N - 2X + 1 to N - X
• ...
• immutable copy version M, which is never released

The problem above is that all these intermediate copies may contain lots of obsolete mutations. Current in-memory mode implementation is that garbage collection is never run for these copies, but it should be.

This ticket is to improve in-memory mode for virtual maps:

• Every copy is first checked if its size exceeds flush threshold
• If so, garbage collection is run for this copy, otherwise no GC
• Then the size of the copy is checked again. If it still exceeds the threshold, the copy is flushed
• Otherwise it is merged to the next version

[artemananiev]

Bumping priority to Critical, since it will need to be backported to 0.58