Add 'active_in' column to pg_replication_slots.

Right now it is visible whether a replication slot is active in any
session, but not in which.  Adding the active_in column, containing the
pid of the backend having acquired the slot, makes it much easier to
associate pg_replication_slots entries with the corresponding
pg_stat_replication/pg_stat_activity row.

This should have been done from the start, but I (Andres) dropped the
ball there somehow.

Author: Craig Ringer, revised by me Discussion:
CAMsr+YFKgZca5_7_ouaMWxA5PneJC9LNViPzpDHusaPhU9pA7g@mail.gmail.com