Rethinking autovacuum.c memory handling

I notice that autovacuum.c calls autovacuum_do_vac_analyze, and
thereby vacuum(), in TopTransactionContext.  This doesn't seem
like a terribly great idea, because it doesn't correspond to what
happens during a manually-invoked vacuum.  TopTransactionContext
will go away when vacuum() commits the outer transaction, whereas
in non-autovac usage, we call vacuum() in a PortalHeapMemory
context that is not a child of TopTransactionContext and is not
at risk of being reset multiple times during the vacuum().  This'd
be a hazard if autovacuum_do_vac_analyze or vacuum did any palloc's
before getting to the main loop.  More generally, I'm not aware of
other cases where we invoke a function in a context that we know
that function will destroy as it executes.

I don't see any live bug associated with this in HEAD, but this behavior
requires a rather ugly (and memory-leaking) workaround in the proposed
patch to allow multiple vacuum target rels.

What I think we should do instead is invoke autovacuum_do_vac_analyze
in the PortalContext that do_autovacuum has created, which we already
have a mechanism to reset once per table processed in do_autovacuum.

The attached patch does that, and also modifies perform_work_item()
to use the same approach.  Right now perform_work_item() has a
copied-and-pasted MemoryContextResetAndDeleteChildren(PortalContext)
call in its error recovery path, but that seems a bit out of place
given that perform_work_item() isn't using PortalContext otherwise.

Comments, objections?

			regards, tom lane

PS: I was disappointed to find out that perform_work_item() isn't
exercised at all in the standard regression tests.