pgsql: Add pinning_backends column to the pg_buffercache extension.

Add pinning_backends column to the pg_buffercache extension.

The new column shows how many backends have a buffer pinned. That can
be useful during development or to diagnose production issues
e.g. caused by vacuum waiting for cleanup locks.

To handle upgrades transparently - the extension might be used in
views - deal with callers expecting the old number of columns.

Reviewed by Fujii Masao and Rajeev rastogi.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f57791985ac3d776cb67e9d69befde538cfdf13b

Modified Files
--------------
contrib/pg_buffercache/Makefile                    |    2 +-
.../pg_buffercache/pg_buffercache--1.0--1.1.sql    |   11 ++++++
contrib/pg_buffercache/pg_buffercache--1.0.sql     |   20 -----------
contrib/pg_buffercache/pg_buffercache--1.1.sql     |   21 +++++++++++
contrib/pg_buffercache/pg_buffercache.control      |    2 +-
contrib/pg_buffercache/pg_buffercache_pages.c      |   37 ++++++++++++++++++--
doc/src/sgml/pgbuffercache.sgml                    |    7 ++++
7 files changed, 76 insertions(+), 24 deletions(-)