Merge pull request #77 from BoresExpress/master

Closes issue LIBZMQ-504
Revert "Fix assertion failed in tcp_listener.cpp:58"

This reverts commit a87aed284e10accc88456498d231628d84d41f6a.
Causes an assertion failure: s != retired_fd (tcp_connecter.cpp:271)
Merge pull request #78 from hintjens/master

Reverted fix for LIBZMQ-504
Closes LIBZMQ-504.

This time correct backport from 3.x branch.