[entropy.db] add direct access support to repository

In latency sensitive code paths, the performance penality caused
by file lock contention and memory cache invalidation is too high.
This problem happens in Rigo, which is extremely latency sensitive.
Since we don't want to crap on the user, a way to solve this is
letting API consumers skip the memory cache and read data directly
from the database store. The trade off is that data may be stale,
incomplete, or invalid, but as long as the consumer is aware of this,
that's fine.