Change session timer to monotonic when possible

Previously the ipmi session was using time.time().  This means
that retries and keepalives could be thrown off by things like ntp
or manual time corrections.

Ideally, we'd use the baked in time.monotonic(), but that doesn't exist
aside from python 3.3

Change-Id: Ia00026cef6df214f9463909309de44767c3752b5