Run checkpointer and bgwriter in crash recovery.

Start up the checkpointer and bgwriter during crash recovery (except in
--single mode), as we do for replication.  This wasn't done back in
commit cdd46c76 out of caution.  Now it seems like a better idea to make
the environment as similar as possible in both cases.  There may also be
some performance advantages.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Tested-by: Jakub Wartak <Jakub.Wartak@tomtom.com>
Discussion: https://postgr.es/m/CA%2BhUKGJ8NRsqgkZEnsnRc2MFROBV-jCnacbYvtpptK2A9YYp9Q%40mail.gmail.com

src/backend/postmaster/checkpointer.c

@@ -10,9 +10,6 @@
  * fill WAL segments; the checkpointer itself doesn't watch for the
  * condition.)
  *
- * The checkpointer is started by the postmaster as soon as the startup
- * subprocess finishes, or as soon as recovery begins if we are doing archive
- * recovery.  It remains alive until the postmaster commands it to terminate.
  * Normal termination is by SIGUSR2, which instructs the checkpointer to
  * execute a shutdown checkpoint and then exit(0).  (All backends must be
  * stopped before SIGUSR2 is issued!)  Emergency termination is by SIGQUIT;