[NTTCP-12] Fix potential locking when waiting on the crossed MVar

[Imported from JIRA. Reported by Facundo Dominguez @facundominguez) as NTTCP-12 on 2015-05-27 20:55:45]
The handler of incomming requests is waiting on the {{crossed}} MVar before accepting an incoming connection.

This wait is potentially indefinite (if the CrossConnectRequest message does not arrive), and it looks unnecessary. At least I cannot come up with any problematic interleaving of events in the absence of it.

We should either remove the {{crossed}} MVar or document what is helping to accomplish (preferrably the problematic ordering of events that is being ruled out).

There is the commit [message|https://github.com/haskell-distributed/network-transport-tcp/commit/6207de60bae7fcf39c108a49bdc152a98abdd277] of the commit that introduces it. Unfortunately, it doesn't explain the "certain ordering of events" that motivates it.

Start with the incoming-request handler and the crossed MVar described in the issue, then read commit 6207de60bae7fcf39c108a49bdc152a98abdd277 for its original context. Determine whether the wait can be removed safely or document the ordering it protects; done means the locking concern is resolved and the rationale is explicit.