Exposing the stats snapshot timestamp to SQL

In a previous thread Tom Lane said:

(I'm also wondering if it'd make sense to expose the stats timestamp
> as a callable function, so that the case could be dealt with
> programmatically as well. But that's future-feature territory.)

(http://www.postgresql.org/message-id/27251.1421684169@sss.pgh.pa.us)

It seemed the appropriate scope for my first submission, and that feature
has been on my wish list for a while, so I thought I'd grab it.

Main purpose of this patch is to expose the timestamp of the current stats
snapshot so that it can be leveraged by monitoring code. The obvious case
is alerting if the stats collector becomes unresponsive. The common use
case is to smooth out graphs which are built from high frequency monitoring
of the stats collector.

The timestamp is already available as part of PgStat_GlobalStats. This
patch is just the boilerplate (+docs & tests) needed to expose that value
to SQL. It shouldn't impact anything else in the server.

I'm not particularly attached to the function name, but I didn't have a
better idea.

The patch should apply cleanly to master.

- Matt K