Minor changes to autovacuum worker: change error handling so that it continues

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.