Refactor Capnproto usage once direct asyncio integration is available

[simon-staal]

pycapn is currently undergoing major development to better integrate with asyncio's event loop - see capnproto/pycapnp#311. This would substantially improve both the performance and simplicity of the server / client code, reducing amount of boilerplate and reliance on magic timeout values. Once the update is available in new major version, the server and client code should be updated to use this new functionality.