Repair some problems in bgwriter start/stop logic. In particular, don't

Repair some problems in bgwriter start/stop logic.  In particular, don't
allow the bgwriter to start before the startup subprocess has finished
... it tends to crash otherwise.  (The same problem may have existed for
the checkpointer, I'm not entirely sure.)  Remove some code that was
redundant because the bgwriter is handled as a member of the backend list.