Cause ShutdownPostgres to do a normal transaction abort during backend

Cause ShutdownPostgres to do a normal transaction abort during backend
exit, instead of trying to take shortcuts.  Introduce some additional
shutdown callback routines to eliminate kluges like having ProcKill
be responsible for shutting down the buffer manager.  Ensure that the
order of operations during shutdown is predictable and what you would
expect given the module layering.