Merge with PostgreSQL, up to the point where the REL9_3_STABLE branch was
created, and 9.4 development started on the PostgreSQL master branch. That
is almost up to 9.3beta2.

Notable upstream changes, from a GPDB point of view:

* LATERAL support. Mostly works in GPDB now, although performance might not
  be very good. LATERAL subqueries, except for degenerate cases that can be
  made non-LATERAL during optimization, typically use nested loop joins.
  Unless the data distribution is the same on both sides of the join, GPDB
  needs to add Motion nodes, and cannot push down the outer query parameter
  to the inner side through the motion. That is the same problem we have
  with SubPlans and nested loop joins in general, but it happens frequently
  with LATERAL. Also, there are a couple of cases, covered by the upstream
  regression tests, where the planner currently throws an error. They have
  been disabled and marked with GPDB_93_MERGE_FIXME comments, and will need
  to be investigated later. Also, no ORCA support for LATERAL yet.

* Materialized views. They have not been made to work in GPDB yet. CREATE
  MATERIALIZED VIEW works, but REFRESH MATERIALIZED VIEW does not. The
  'matviews' test has been temporarily disabled, until that's fixed. There
  is a GPDB_93_MERGE_FIXME comment about this too.

* Support for background worker processes. Nothing special was done about
  them in the merge, but we could now make use of them for all the various
  GPDB-specific background processes, like the FTS prober and gpmon
  processes.

* Support for writable foreign tables was introduced. I believe foreign
  tables now have all the same functionality, at a high level, as external
  tables, so we could start merging the two concepts. But this merge commit
  doesn't do anything about that yet, external tables and foreign tables
  are still two entirely different beasts.

* A lot of expected output churn, thanks to a few upstream changes. We no
  longer print a NOTICE on implicitly created indexes and sequences (commit
  d7c73484), and the rules on when table aliases are printed were changed
  (commit 11e13185).

* Caught up to a bunch of features that we had already backported from 9.3:
  data page checksums, numeric datatype speedups, COPY FROM/TO PROGRAM, and
  pg_upgrade as whole.

A couple of other noteworthy changes:

* contrib/xlogdump utility is removed, in favor of the upstream
  contrib/pg_xlogdump utility.

* Removed "idle session timeout" hook. The current implementation was badly
  broken by upstream refactoring of timeout handling (commit f34c68f0).
  We'll probably need to re-introduce it in some form, but it will look
  quite different, to make it fit more nicely with the new timeout APIs.
Co-authored-by: NAshwin Agrawal <aagrawal@pivotal.io>
Co-authored-by: NAsim R P <apraveen@pivotal.io>
Co-authored-by: NDavid Kimura <dkimura@pivotal.io>
Co-authored-by: NEkta Khanna <ekhanna@pivotal.io>
Co-authored-by: NHeikki Linnakangas <hlinnakangas@pivotal.io>
Co-authored-by: NJacob Champion <pchampion@pivotal.io>
Co-authored-by: NJinbao Chen <jinchen@pivotal.io>
Co-authored-by: NKalen Krempely <kkrempely@pivotal.io>
Co-authored-by: NPaul Guo <paulguo@gmail.com>
Co-authored-by: NRichard Guo <guofenglinux@gmail.com>
Co-authored-by: NShaoqi Bai <sbai@pivotal.io>