add special attribute 'ts' to a JSON property reflecting
/sim/time/elapsed-sec
always add timestamp for listener properties
add timestamp for json requests if req param t=y is set
* mongoose updated to 5.3
* first stab at implementing websockets, here a property change listener
websocket. This websocket is at ws://yourhost:yourport/PropertyListener
see FGDATA/Docs/gui/radio.html for an example