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: bfccd1945c2acd6a1e987856893e427d663f79a9
  1. v23/
  2. x/
  3. .gitignore
  4. AUTHORS
  5. CONTRIBUTORS
  6. LICENSE
  7. PATENTS
  8. VERSION