Move more bgworker code to bgworker.c; also, some renaming.

Robert Haas <rhaas@postgresql.org>

Commit: 2dee7998f93062e2ae7fcc9048ff170e528d1724
Author: Robert Haas <rhaas@postgresql.org>
Date: 2013-08-16T19:31:28Z
Releases: 9.4.0
Move more bgworker code to bgworker.c; also, some renaming.

Per discussion on pgsql-hackers.

Michael Paquier, slightly modified by me.  Original suggestion
from Amit Kapila.

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