Smooth reporting of commit/rollback statistics.

If a connection committed or rolled back any transactions within a
PGSTAT_STAT_INTERVAL pacing interval without accessing any tables,
the reporting of those statistics would be held up until the
connection closed or until it ended a PGSTAT_STAT_INTERVAL interval
in which it had accessed a table.  This could result in under-
reporting of transactions for an extended period, followed by a
spike in reported transactions.

While this is arguably a bug, the impact is minimal, primarily
affecting, and being affected by, monitoring software.  It might
cause more confusion than benefit to change the existing behavior
in released stable branches, so apply only to master and the 9.4
beta.

Gurjeet Singh, with review and editing by Kevin Grittner,
incorporating suggested changes from Abhijit Menon-Sen and Tom
Lane.