vinyl: delete runs compacted during join immediately

We keep run files corresponding to (at least) the last snapshot, because
we need them for backups and replication. Deletion of compacted run
files is postponed until the next snapshot. As a consequence, we don't
delete run files created on a replica during the join stage. However, in
contrast to run files created during normal operation, these are pure
garbage and should be deleted right away. Not deleting them can result
in depletion of disk space, because vinyl has quite high write
amplification by design.

We can't write a functional test for this, because there's no way to
guarantee that compaction started during join will finish before join
completion - if it doesn't, compacted runs won't be removed, because
they will be assigned to the snapshot created by join.

Closes #3162