Avoid archiving XLOG_RUNNING_XACTS on idle server

If archive_timeout > 0 we should avoid logging XLOG_RUNNING_XACTS if idle.

Bug 13685 reported by Laurence Rowe, investigated in detail by Michael Paquier,
though this is not his proposed fix.
20151016203031.3019.72930@wrigleys.postgresql.org

Simple non-invasive patch to allow later backpatch to 9.4 and 9.5