Fix units and default values for heartbeats options

Set the ZMQ_HEARTBEAT_TIMEOUT to default to the value of
ZMQ_HEARTBEAT_IVL if it's not explicitly set.
Change the units of ZMQ_HEARTBEAT_TTL to milliseconds in the API
and round down to the nearest decisecond so that all the options
are using the same units.
Make the maximum heartbeat TTL match the spec (6553 seconds)

src/stream_engine.hpp

@@ -219,6 +219,7 @@ namespace zmq
         bool has_ttl_timer;
         bool has_timeout_timer;
         bool has_heartbeat_timer;
+        int heartbeat_timeout;
 
         // Socket
         zmq::socket_base_t *socket;