Use timeutils.utcnow() throughout the code

timeutils.utcnow() should be used instead of direct calls to
datetime.datetime.utcnow() to make it easy to override its return
value in tests.

Add a hacking rule to prevent regressions.

Fixes bug 1200141.

Change-Id: I170dbd7c9093bd627e2a0d5984b7ad1bf105c8d5
Merge "Use timeutils.utcnow() throughout the code"