vinyl: remove vylog newer than snap in casual recovery

As a follow-up to the previous patch, let's check also emptiness of the
vylog being removed. During vylog rotation all entries are squashed
(e.g. "delete range" annihilates "insert range"), written to the new
vylog and at the end of new vylog SNAPSHOT marker is placed. If the last
entry in the vylog is SNAPSHOT, we can safely remove it without
hesitation.  So it is OK to remove it even during casual recovery
process. However, if it contains rows after SNAPSHOT marker, removal of
vylog may cause data loss. In this case we still can remove it only in
force_recovery mode.

Follow-up #5823