Change compute updates from periodic to on demand

[OpenStack Infra]

https://review.openstack.org/101008
commit f44b4faacd2a21fa27751a32e7a908022ab089ad
Author: Don Dugger <email address hidden>
Date: Wed Jun 18 13:13:11 2014 -0600

    Change compute updates from periodic to on demand

    Currently, all compute nodes update status info in the DB on a periodic
    basis (the period is currently 60 seconds). Given that the status of
    the node only changes at specific points (mainly image
    creation/destruction) this leads to significant DB overhead on a large
    system. This BP changes the update mechanism to only update the DB when
    a node state changes by keeping an old copy of the state info and
    only updating the DB if the new state info is different.

    Docimpact - check to see if there's a documentation impact to this change.

    Change-Id: I19c174488b61977358a4d617cc4b590e317e665a
    Implements: blueprint on-demand-compute-update