store/leveldb: adding mutexes

Wrapping db, snapshot and transaction with mutexes,
adding a bunch of tests.

This change also removes verrors from internal storage engine
for consistency: we should either use verrors everywhere or not
use them at all. As discussed with Adam offline, we should
probably start from the latter and switch to verrors later.

Change-Id: I38bf9760b35c40943225629c37e7d41ec8c30085
21 files changed
tree: 7f206d92a75a1a29c29c012ad484fa3252e49d45
  1. services/