Do not track active reservations

Do not track active reservations

Reservations have a transient nature: a reservation lifespan
typically begins and ends with a single request.
Therefore tracking reserved amounts for each tenant and resource
is not nearly as efficient as tracking resource usage.
Indeed it is fairly easy to verify that the overhead for tracking
reserved amounts is much greater than the one needed for counting
active reservations for each tenant and resource.

This patch removes the logic for tracking reservations, and
replaces it with an explicit count of active reservations.

Please note that this patch does not adjust accordingly the
ResourceUsage DB model. This will be done in a separate patch with
an expand migration; this should avoid most merge conflicts before
the final patch for restoring reservation logic merges.

Related-Blueprint: better-quotas

Change-Id: Ib5e3bd61c1bc0fc8a5d612dae5c1740a8834a980