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.
Version 0.9.1

https://github.com/cms-dev/cms/wiki/CMS-0.9.1-RELEASE-NOTES