Campaign: Add accounting reader and writer

Campaign Leader: Tim Freeman

Technologies: VWS

Objective:
Add accounting support to VWS, logically separating the reading and writing
activities to allow for alternative plugins for one or both.

Benefits:
This allows clients and administrators to get usage information as well as
allows for the authorization callout to inspect previous usage when making a
decision.

Implementation details/tasks:
1) Add reader and writer plugins
2) Add default database backed implementation of each
3) Call writer at appropriate places (create/destroy)
4) Add event logging to the default implimentation, output to a file that logs
all create and destroy events with workspace ID, DN of caller, and the
requested/used duration (as appropriate)
5) Add "current reservations" logging to a file that provides a view of the
current deployments to the administrator without needing to go the DB
6) Reader plugin cannot exist without writer
7) Include charge granularity, for backends that implement reservations/pricing
by blocks of minutes rather than minute by minute (for example a certain
backend we use treats 1 to 60 minutes as 60)