A behaviour module for implementing a server, which maintains a local tree (key-value map) as its internal state. A source is supposed to be mounted on a TreeServer at a path via the TreeServer.mount_source/1 function, normally during the c:init/1 callback. It is then supposed to do two things:

• Reply to query requests by returning the local tree in a mounted state (done by the handle_query functions in this module)
• Notify the hosting TreeServer about updates in the local tree via the TreeServer.notify/2 function