Enable configurable log of autovacuum actions. Initial patch from Simon

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: ef23a77441b94d1e2fde3f359d2e49703c70c9ca
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2007-04-18T16:44:18Z
Releases: 8.3.0
Enable configurable log of autovacuum actions.  Initial patch from Simon
Riggs, additional code and docs by me.  Per discussion.

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