Move isolationtester's is-blocked query into C code for speed.

Commit 4deb41381 modified isolationtester's query to see whether a
session is blocked to also check for waits occurring in GetSafeSnapshot.
However, it did that in a way that enormously increased the query's
runtime under CLOBBER_CACHE_ALWAYS, causing the buildfarm members
that use that to run about four times slower than before, and in some
cases fail entirely.  To fix, push the entire logic into a dedicated
backend function.  This should actually reduce the CLOBBER_CACHE_ALWAYS
runtime from what it was previously, though I've not checked that.

In passing, expose a SQL function to check for safe-snapshot blockage,
comparable to pg_blocking_pids.  This is more or less free given the
infrastructure built to solve the other problem, so we might as well.

Thomas Munro

Discussion: https://postgr.es/m/20170407165749.pstcakbc637opkax@alap3.anarazel.de
