Allow execution and discard of undo by background workers.

Allow execution and discard of undo by background workers.

Undo launcher is responsible for launching the workers iff there is some
work available in one of the work queues and there are more workers
available.  The worker is launched to handle requests for a particular
database.

The discard worker is responsible for discarding the undo log of
transactions that are committed and all-visible or are rolled-back.  It
also registers the request for aborted transactions in the work queues.
It iterates through all the active logs one-by-one and tries to discard the
transactions that are old enough to matter.

We don't allow any transaction older than 2^31 to have pending undo actions.
Also, we have a hard limit on the number of transactions that can have
pending undo which is proportional to pending_undo_queue_size.

Amit Kapila, Dilip Kumar and Kuntal Ghosh with inputs from Andres Freund,
Robert Haas and Thomas Munro.