#2824 Websocket Property Subscription Enhancements and Fixes

[Pat Callahan]

Websockets in flightgear/Network/http have several issues recently discussed on the mailing list

After extensive discussions, false starts, and a lot of investigation of the websocket code, This issue is complex enough to do the work in stages under separate tickets describing specific issues or enhancements.

This ticket will cover only one small change , but I'm including a list of the remaining ideas for future followup.

1. Fix a bug in MirrorPropertyTreeWebsocket that drops unsent changes . This bug was actually noted during coding for Ticket 2850, for item 2 below, but will be a separate change, just the fix.

The remaining ideas should be opened under separate tickets.

For MirrorPropertyTreeWebsocket

1. Expand the MirrorPropertyTreeWebsocket's protocol with additional handlers
   similar to those in PropertyChangeWebsocket. see: https://sourceforge.net/p/flightgear/codetickets/2850/

2. Allow changing the default update rate of MirrorPropertyChangeWebsocket.

4 Modify the MirrorPropertTreeWebsocket to only send changes to doubles
or floats if the change exceeds a minimum change amount.

1. Allow setting the update rate and minimum change amount from the
   command line, configuration file, or websocket request.

For Phi:

1. Update Phi, replacing all PropertyChangeWebsockets with
   MirrorPropertyTreeWebsockets.

2. Update Phi to be entirely reactive. Property Tree Display would
   update itself; expanding a node adds listeners, and collapsing removes them.

3. Upgrade Mongoose to the latest version. The current version of
   Mongoose contains breaking changes, so it is a complex task.

4. Consider alternatives to Mongoose. Knockout.js (the latest Knockout.js version
   is from 2019.) and any necessary upgrades to other javascript libraries:
   leaflet, jquery-ui-themes, etc.

For Simgear Props:

1. Investigate memory leaks as noted in props.cxx