Stamp HEAD as 9.4devel.

Let the hacking begin ...
Stamp shared-library minor version numbers for 9.4.
Update RELEASE_CHANGES to describe library version bumping more fully.
Add :client_id automatic variable for custom pgbench scripts.

This makes it easier to write custom scripts that have different logic for
each client.

Gurjeet Singh, with some changes by me.
Merge branch 'master' into filter
Merge branch 'filter' into within_group