Remove event related memory leak
Remove event related memory leak

It was possible for a session to accumulate
a plurality of events per raw_command issued.
Alter the behavior of session to set and discard
*all* pending events on exit.  This means that if
there are multiple commands waiting, then things
will be a tad more work than is strictly necessary,
but it's the simplest way to eliminate the leak,
the amount of senseless work is pretty small even
in some pretty tortuous cases, and it only has
any impact when many commands are being hit against
same target concurrently without coordination, which
is rare.

Change-Id: I3f023cee6fc7bb537c1a508d3113f2ff238828de

openstack/pyghmi,stackforge/pyghmi,openstack/pyghmi