REQ sockets drop replies from unasked peers.

* 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.
* Enable test in test_spec_req.
Fix coding style in spec tests.
Merge pull request #608 from ckamm/req-drops-unwanted

REQ sockets drop replies from unasked peers.
Merge pull request #613 from ckamm/coding-style-spec-tests

Fix coding style in spec tests.
Fix intermittent failure in test_spec_req.

There still is a problem when a stale peer's message
arrives at the REQ just after a request was sent to that peer.
Merge pull request #614 from ckamm/fix-spec-req-fail

Fix intermittent failure in test_spec_req.