Use FileHandle for reading/writing in JSONRPCConnection

There's a long standing issue with `DispatchIO.read` on Windows that
causes a continual spin. `DispatchIO` is more complex than what we need
here, so switch to `FileHandle.readabilityHandler` for reading and
`FileHandle.write` for writing.

Given there's been issues with `FileHandle.availableData` and we can't
read to EOF, the reads are instead broken up into chunks - reading one
byte at a time for the header and then the entire contents after we have
the content length. The header read could be a little more optimized,
but reading a single byte allows for better error handling in the case
of an invalid header.

Using the `FileHandle`s directly also allows us to simplify a lot of the
surrounding code, as there's no need for a `DispatchGroup` or
`sendQueue`.

A further change could switch `JSONRPCConnection` to return an
`AsyncStream` for the received messages rather than being passed a
`MessageHandler` (skipping for now as this change is already large
enough and all existing clients already use `MessageHandler`).

Many thanks to @roman-bcny for starting this change.

Author: bnbarham