#59 vusage daemon should use auth module for lastauth time

[Matt Brookings]

The vusage daemon checks for a lastauth file on the disk to determine last authentication time for userstore aging. It should call the backend module API.
To do this, the user portion of the code should take up USERSTORE_AGE_MAX and USERSTORE_DAY_AGE. Enforcement of these two configurations should not be done under the userstore.