Implement the /api/stats endpoint
Move the Stats RPC class/handler into it's own file, StatsRpc.java
Add serializer format calls for statistics to be emitted as JSON objects

Signed-off-by: Chris Larsen <clarsen@euphoriaaudio.com>
Add missing TreeRpc "enabled" query string parse code

Signed-off-by: Chris Larsen <clarsen@euphoriaaudio.com>