pgsql: Minor changes to autovacuum worker: change error handling so that

Log Message:
-----------
Minor changes to autovacuum worker: change error handling so that it continues
with the next table on schedule instead of exiting, in all cases instead of
just on query cancel.

Add a errcontext() line indicating the activity of the worker to the error
message when it is cancelled.

Change the WorkerInfo struct to contain a pointer to the worker's PGPROC
instead of just the PID.

Add forgotten post-auth delays, per Simon Riggs.  Also to autovac launcher.

Modified Files:
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    pgsql/src/backend/postmaster:
        autovacuum.c (r1.61 -> r1.62)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.61&r2=1.62)