create skeleton files for event storage backends

this patch adds support to configure an event specific database.
the code currently calls existing metering/event mixed code so
basically both metering and event db share the same code.

segregation of code will be done in subsequent patches for each
driver individually.

Change-Id: I22871737cf297d1bfff6c17a7eaf9333c6415ffd
Partially-Implements: blueprint dedicated-event-db
Merge "create skeleton files for event storage backends"