Async: remove support for binary responses.

They're not used and simplifying the protocol may help towards
restructuring the IO model underlying CMS.
Async: remove length at the beginning of RPC messages.

This fix a nasty bug that would have been triggered by having
a JSON length that contains the bytes '\r\n', which are used for
message termination. Fortunately our messages are always short
enough that, to my knowledge, this bug has never been triggered.