* Update documentation to async code (#331)
This commit updates the documentation to the latest changes added
with pycapnp 2.0.0.
* Remove non existing classes/functions from the reference documentation
* Adapt the quickstart to the latest changes. Mainly to new rpc handling,
that now exlusively is done through asyncio.
* DOC: Add section about send and receive messages over a socket
Since #313 it is possible to read and write messages over a socket.
This commit adds a small section for read and write in the quickstart.
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