Introduce _vinyl_deferred_delete system space

The space is a blackhole. It will be used for writing deferred DELETE
statements generated by vinyl compaction tasks to WAL so that we can
recover deferred DELETEs that hadn't been dumped to disk before the
server was restarted.

Since the new space doesn't depend on other system spaces, let's assign
the minimal possible id to it, i.e. 257.

Needed for #2129