pgsql: Add 'active_in' column to pg_replication_slots.

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(at)mail(dot)gmail(dot)com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d811c037cea2e929ee5823aab6e1bca0df8be245

Modified Files
--------------
contrib/test_decoding/expected/ddl.out | 4 ++--
doc/src/sgml/catalogs.sgml | 10 ++++++++++
doc/src/sgml/logicaldecoding.sgml | 8 ++++----
src/backend/catalog/system_views.sql | 1 +
src/backend/replication/slot.c | 30 ++++++++++++++++--------------
src/backend/replication/slotfuncs.c | 13 +++++++++----
src/include/catalog/pg_proc.h | 2 +-
src/include/replication/slot.h | 4 ++--
src/test/regress/expected/rules.out | 3 ++-
9 files changed, 47 insertions(+), 28 deletions(-)