Optimize timeutils.utcnow_ts()

This patch short-circuits the use of utcnow() when it is not
needed, resulting in a ~7x increase in performance, putting the
cost of the call in the nanosecond range.

Change-Id: I7c89140935bd8c9a702deb8ff5bcb892624ff76b
Merge "Optimize timeutils.utcnow_ts()"