LU-795 osd api: Commit callback per transaction

- 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
- use th_sync to mark sync operations

Change-Id: If5f8f2a6d3cd2f3e77fd13c802213a181043a2d7
Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1621


Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>