Fix memory leak in LogStandbySnapshot().

Fix memory leak in LogStandbySnapshot().

The array allocated by GetRunningTransactionLocks() needs to be pfree'd
when we're done with it.  Otherwise we leak some memory during each
checkpoint, if wal_level = hot_standby.  This manifests as memory bloat
in the checkpointer process, or in bgwriter in versions before we made
the checkpointer separate.

Reported and fixed by Naoya Anzai.  Back-patch to 9.0 where the issue
was introduced.

In passing, improve comments for GetRunningTransactionLocks(), and add
an Assert that we didn't overrun the palloc'd array.