pgsql: Add support for REFRESH MATERIALIZED VIEW CONCURRENTLY.

Add support for REFRESH MATERIALIZED VIEW CONCURRENTLY.

This allows reads to continue without any blocking while a REFRESH
runs.  The new data appears atomically as part of transaction
commit.

Review questioned the Assert that a matview was not a system
relation.  This will be addressed separately.

Reviewed by Hitoshi Harada, Robert Haas, Andres Freund.
Merged after review with security patch f3ab5d4.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/cc1965a99bf87005f431804bbda0f723887a04d6

Modified Files
--------------
doc/src/sgml/mvcc.sgml                          |    3 +-
doc/src/sgml/ref/refresh_materialized_view.sgml |   34 +-
src/backend/commands/cluster.c                  |   27 +-
src/backend/commands/matview.c                  |  524 +++++++++++++++++++++--
src/backend/commands/tablecmds.c                |    3 +-
src/backend/executor/execMain.c                 |   10 +-
src/backend/executor/nodeModifyTable.c          |    5 +-
src/backend/nodes/copyfuncs.c                   |    1 +
src/backend/nodes/equalfuncs.c                  |    1 +
src/backend/parser/gram.y                       |    7 +-
src/bin/psql/tab-complete.c                     |   17 +
src/include/commands/cluster.h                  |    3 +-
src/include/commands/matview.h                  |    2 +
src/include/nodes/parsenodes.h                  |    1 +
src/test/regress/expected/matview.out           |   38 +-
src/test/regress/sql/matview.sql                |   29 +-
16 files changed, 646 insertions(+), 59 deletions(-)