Aim to be easier for clients to use, because to do peer
discovery we must act as a client to other servers.
Split out JSON session concept from the asyncio protocol
concept. This makes the JSON RPC support more easily
testable and usable as a stand-alone library.
In addition, support JSON RPC v1 and v2, and auto-detection
of peer's version.
Apart from the flush on shutdown and the flush when caught up,
neither of which matter, this makes flushes asynchronous.
Also, block processing for reorgs is now asynchronous.
This also removes the FORCE_REORG debug envvar; I want to
put that into the RPC interface.
Closes#102
