REQ sockets drop replies from unasked peers.

Please don't merge this as-is, it is a request for review and needs cleanup.

Commit message:
* Add lb_t::sendpipe() that returns the pipe that was used for sending,
  similar to fq_t::recvpipe().
* Add forwarder functions to dealer_t to access these two.
* Add logic to req_t to ignore replies on pipes that are not the one
  where the request was sent.

req.cpp contains several comments of the style "// ###" that I'd like feedback on.

The test case for this code is in https://github.com/zeromq/libzmq/pull/606 .