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.
Rework 'MOVE ALL' to 'ALTER .. ALL IN TABLESPACE'

As 'ALTER TABLESPACE .. MOVE ALL' really didn't change the tablespace
but instead changed objects inside tablespaces, it made sense to
rework the syntax and supporting functions to operate under the
'ALTER (TABLE|INDEX|MATERIALIZED VIEW)' syntax and to be in
tablecmds.c.

Pointed out by Alvaro, who also suggested the new syntax.

Back-patch to 9.4.
merge master