Restructure autovacuum in two processes: a dummy process, which runs

Restructure autovacuum in two processes: a dummy process, which runs
continuously, and requests vacuum runs of "autovacuum workers" to postmaster.
The workers do the actual vacuum work.  This allows for future improvements,
like allowing multiple autovacuum jobs running in parallel.

For now, the code keeps the original behavior of having a single autovac
process at any time by sleeping until the previous worker has finished.