make 'tx.commit-ok' indicate responsibility transfer

This is now very close to the previous tx semantics.
The downsides w.r.t. the previous state of this branch are
- a bit more code, though only in the channel
- tx and confirm mode no longer compose
- tx always carries the cost of confirms

Implementation notes:
- The channel must remain active while a commit waits for confirms
  from queues to trickle in. We achieve this by recording the fact
  that we have a pending commit.
- The trigger for sending the commit-ok is that the number of pending
  confirms drops to zero and we have a pending commit.
- We check for that condition in three places:
  a) at the point of commit (in case the tx contains no publishes or
     all confirmations happen as part of delivering the messages to
     queues)
  b) where we would normally send basic.acks
  c) where we would normally send basic.nacks
- we are re-using the same logic/state as for 'proper' confirms,
  except we suppress the sending of acks/nacks
- handling the failure case is slightly awkward. We record a tx as
  'failed' as soon as we encounter the 'nack' case. Subsequently, when
  the aforementioned triggering condition is met instead of sending a
  tx.commit-ok we send a precondition_failed error. But we can't just
  employ rabbit_misc:protocol_error here since that only works in the
  context of handling an AMQP method, which is only the case for
  (a). So instead we drop to a slightly lower API level and rely on
  the fact precondition_failed is a channel-level, rather than
  connection-level error.

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