NEO did not ensure that all data and metadata are written on disk before
tpc_finish, and it was for example vulnerable to ENOSPC errors.
In other words, some work had to be moved to tpc_vote:

- In tpc_vote, all involved storage nodes are now asked to write all metadata
  to ttrans/tobj and _commit_. Because the final tid is not known yet, the tid
  column of ttrans and tobj now contains NULL and the ttid respectively.

- In tpc_finish, AskLockInformation is still required for read locking,
  ttrans.tid is updated with the final value and this change is _committed_.

- The verification phase is greatly simplified, more reliable and faster. For
  all voted transactions, we can know if a tpc_finish was started by getting
  the final tid from the ttid, either from ttrans or from trans. And we know
  that such transactions can't be partial so we don't need to check oids.

So in addition to minimizing the risk of failures during tpc_finish, we also
fix a bug causin...