pgxn/walproposer: refactor to allow graceful exit

[qsliu2017]

I notice there is a comment in pgxn/neon/walproposer_pg.c::assign_neon_safekeepers. Is there any plan? If not, might I work on it?

	/*
	 * TODO: restarting through FATAL is stupid and introduces 1s delay before
	 * next bgw start. We should refactor walproposer to allow graceful exit
	 * and thus remove this delay. XXX: If you change anything here, sync with
	 * test_safekeepers_reconfigure_reorder.
	 */
	if (!safekeepers_cmp(oldval, newval_copy))
	{
		wpg_log(FATAL, "restarting walproposer to change safekeeper list from %s to %s",
				wal_acceptors_list, newval);
	}