Sending volume IO usage broken

This is my fault, I didn't test the sending of notifications
enough when fixing bug 1182102, when the volume IO usage notifications
were been sent too often. This was merged as part of
https://review.openstack.org/#/c/29915

The issue was that the code worked for the first IO usage event (the
one I looked at). Then when constructing the event payload for the second
and following IO usage events the code path ended up comparing a datetime
object from the db with a cached string representation of the last_refreshed
date that was passed into the conductor via the RPC.

Since last_refreshed argument to vol_usage_update in the db layer is
not needed and is causing these issues. I have removed last_refreshed
argument from the db layer and deprecated it from the conduction API.

Change-Id: I2030eb7912c56134ea688a6e8bbfcdeddca28307
Merge "Sending volume IO usage broken"