Problem: invoking the conditional variable for zero time is expensive

Solution: for zero timeout, unlock and relock immediately instead of timedwait

src/mailbox_safe.cpp

@@ -99,12 +99,20 @@ int zmq::mailbox_safe_t::recv (command_t *cmd_, int timeout_)
     if (_cpipe.read (cmd_))
         return 0;
 
+    //  If the timeout is zero, it will be quicker to release the lock, giving other a chance to send a command
+    //  and immediately relock it.
+    if (timeout_ == 0) {
+        _sync->unlock ();
+        _sync->lock ();
+    } 
+    else {
         //  Wait for signal from the command sender.
         int rc = _cond_var.wait (_sync, timeout_);
         if (rc == -1) {
             errno_assert (errno == EAGAIN || errno == EINTR);
             return -1;
         }
+    }
 
     //  Another thread may already fetch the command
     const bool ok = _cpipe.read (cmd_);