pgsql: Add simple VACUUM progress reporting.

Add simple VACUUM progress reporting.

There's a lot more that could be done here yet - in particular, this
reports only very coarse-grained information about the index vacuuming
phase - but even as it stands, the new pg_stat_progress_vacuum can
tell you quite a bit about what a long-running vacuum is actually
doing.

Amit Langote and Robert Haas, based on earlier work by Vinayak Pokale
and Rahila Syed.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/c16dc1aca5e01e6acaadfcf38f5fc964a381dc62

Modified Files
--------------
doc/src/sgml/monitoring.sgml         | 207 +++++++++++++++++++++++++++++++++++
src/backend/catalog/system_views.sql |  18 +++
src/backend/commands/vacuumlazy.c    |  78 +++++++++++++
src/backend/postmaster/pgstat.c      |  29 +++++
src/include/catalog/catversion.h     |   2 +-
src/include/commands/progress.h      |  37 +++++++
src/include/pgstat.h                 |   2 +
src/test/regress/expected/rules.out  |  22 ++++
8 files changed, 394 insertions(+), 1 deletion(-)