Don't repeatedly register cache callbacks in pgoutput plugin.

Don't repeatedly register cache callbacks in pgoutput plugin.

Multiple cycles of starting up and shutting down the plugin within a
single session would eventually lead to "out of relcache_callback_list
slots", because pgoutput_startup blindly re-registered its cache
callbacks each time.  Fix it to register them only once, as all other
users of cache callbacks already take care to do.

This has been broken all along, so back-patch to all supported branches.

Shi Yu

Discussion: https://postgr.es/m/OSZPR01MB631004A78D743D68921FFAD3FDA79@OSZPR01MB6310.jpnprd01.prod.outlook.com