PATCH: Add 'pid' column to pg_replication_slots

The attached patch adds a 'pid' column to pg_replication_slots, so it's
possible to associate an active slot with the pg_stat_replication entry
that describes the walsender using the slot.

If a user backend (or bgworker) is using the slot over the SQL interface,
the 'pid' column will correspond to the pg_stat_activity entry for that
backend instead. After all, both it and pg_stat_replication are views
over pg_stat_get_activity() anyway.

Detailed rationale in patch.

Please consider this for 9.5. It's a pretty light patch, and it'd be good
to have it in place to ease monitoring of slot-based replication.

Note that logical replication walsenders are broken in HEAD so testing this
using the test_decoding module and pg_recvlogical will crash the walsender.
This is a pre-existing bug; see
http://www.postgresql.org/message-id/CAB7nPqQSdx7coHk0D6G=mkJntGYjXPDw+PWisKKSsAeZFTskvg@mail.gmail.com
and
http://www.postgresql.org/message-id/CAMsr+YEh50r70+hP+w=rCzEuenoQRCNMDA7PmRSK06Ro9r=9sA@mail.gmail.com
)

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services