This involved a rewrite of the JSON RPC layer.
I took the opportunity to clean up the handling of requests
in general. It should now be easy to return nice help from
the docstrings.
Closes issue #99
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