pgsql: Add support for multiple versions of an extension and ALTER EXTE

Add support for multiple versions of an extension and ALTER EXTENSION UPDATE.

This follows recent discussions, so it's quite a bit different from
Dimitri's original.  There will probably be more changes once we get a bit
of experience with it, but let's get it in and start playing with it.

This is still just core code.  I'll start converting contrib modules
shortly.

Dimitri Fontaine and Tom Lane

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/1214749901fc3c66732cfd9f276b989635c01360

Modified Files
--------------
contrib/pg_upgrade_support/pg_upgrade_support.c |    9 +-
doc/src/sgml/catalogs.sgml                      |    2 +-
doc/src/sgml/extend.sgml                        |  175 +++++-
doc/src/sgml/ref/alter_extension.sgml           |   37 +-
doc/src/sgml/ref/create_extension.sgml          |   52 ++-
src/backend/commands/extension.c                |  753 +++++++++++++++++++++--
src/backend/nodes/copyfuncs.c                   |   14 +
src/backend/nodes/equalfuncs.c                  |   12 +
src/backend/parser/gram.y                       |   52 ++-
src/backend/tcop/utility.c                      |   10 +
src/bin/pg_dump/pg_dump.c                       |   10 +-
src/bin/psql/tab-complete.c                     |    2 +-
src/include/catalog/pg_extension.h              |    6 +-
src/include/commands/extension.h                |    2 +
src/include/nodes/nodes.h                       |    1 +
src/include/nodes/parsenodes.h                  |    8 +
16 files changed, 1027 insertions(+), 118 deletions(-)