[grpc] investigate DB lock files

[okayzed]

The DB lock files will no longer work as expected when using multiple queries or ingestions in a single sybil process, because they use the PID to coordinate and figure out the owner of the lock.

This task is to adjust the locks: ingest, digest, info and cache (see the sybil implementation page for what they do) to be protected internally against contention from other threads. It might mean adding a mutex or otherwise solving this issue.

(refs #38)