Improve memory management in autovacuum.c.

Invoke vacuum(), as well as "work item" processing, in the PortalContext
that do_autovacuum() has manufactured, which will be reset before each
such invocation.  This ensures cleanup of any memory leaked by these
operations.  It also avoids the rather dangerous practice of calling
vacuum() in a context that vacuum() itself will destroy while it runs.
There's no known live bug there, but it's not hard to imagine introducing
one if we leave it like this.

Tom Lane, reviewed by Michael Paquier and Alvaro Herrera

Discussion: https://postgr.es/m/13849.1506114543@sss.pgh.pa.us