Only start the heartbeat timer when the underlying mechanism is ready to stop…

Only start the heartbeat timer when the underlying mechanism is ready to stop cipher streams from trying to encrypt a heartbeat before the handshake has completed. Addresses issue #1601.

src/stream_engine.cpp

@@ -712,11 +712,6 @@ bool zmq::stream_engine_t::handshake ()
         }
         next_msg = &stream_engine_t::next_handshake_command;
         process_msg = &stream_engine_t::process_handshake_command;
-
-        if(options.heartbeat_interval > 0) {
-            add_timer(options.heartbeat_interval, heartbeat_ivl_timer_id);
-            has_heartbeat_timer = true;
-        }
     }
 
     // Start polling for output if necessary.
@@ -824,6 +819,11 @@ void zmq::stream_engine_t::zap_msg_available ()
 
 void zmq::stream_engine_t::mechanism_ready ()
 {
+    if (options.heartbeat_interval > 0) {
+        add_timer(options.heartbeat_interval, heartbeat_ivl_timer_id);
+        has_heartbeat_timer = true;
+    }
+
     if (options.recv_identity) {
         msg_t identity;
         mechanism->peer_identity (&identity);