Reserve replication slots specifically for REPACK

Reserve replication slots specifically for REPACK

Add a new GUC max_repack_replication_slots, which lets the user reserve
some additional replication slots for concurrent repack (and only
concurrent repack).  With this, the user doesn't have to worry about
changing the max_replication_slots in order to cater for use of
concurrent repack.

(We still use the same pool of bgworkers though, but that's less
commonly a problem than slots.)

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133@gmail.com>
Discussion: https://postgr.es/m/202604012148.nnnmyxxrr6nh@alvherre.pgsql