Use our SQLite catalogs for the pgcopydb sentinel table.

The main use-case for the sentinel table is to communicate progress made by
the three independent worker processes for stream, transform, and replay.
It is also needed to communicate new startpos, endpos and apply values with
those processes from another pgcopydb (interactive) command, a process
that's not in the process tree.

This was previously solved by creating and using a pgcopydb.sentinel table
on the source database. Now that we have a SQLite database internal to the
pgcopydb run, we can use that instead.