Port from orion branch to fix issue with synchronous SEQ update found by mds_survey runs.

• ability to add commit callback per transaction in addition to per-device
  hooks. Now it is much simpler if only commit callback is needed.
• rewrite commit callbacks for last_commit and new_client, add commit callback
  in seq manager
• cleanup not-needed code: old commit callbacks, txn_keys
• remove osd od_env_for_commit environment and env param from commit callbacks