vinyl: optimize deletion of compacted run files

On completion of compaction tasks we remove compacted run files created
after the last checkpoint immediately to save disk space. In order to
perform this optimization we compare the unused runs' dump LSN with the
last checkpoint's one.

But during replica's initial JOIN stage we set the LSN of all rows
received from remote master to 0 (see
box/box.cc/boostrap_journal_write). Considering that the LSN of an
initial checkpoint is also 0, our optimization stops working, and we
receive a huge disk space usage spike (as the unused run files will
only get removed when garbage collection occurs).

We should check the vinyl space engine's status and perform
our optimization unconditionally if we are in replica's initial JOIN
stage.

Closes #6568

(cherry picked from commit 599f0c13)