Remove the synchronous RPC mode

[LasseBlaauwbroek]

Done in this PR:

• Removed the possibility of running RPC without asyncio
• All capability methods now have to be async. Calling methods can no longer be done using .wait(). Chaining can no longer be done using then()
• Removed VoidPromise and most of Promise. Only RemotePromise remains as a major class, for pipelining purposes.
• Removed the possibility of starting a server/client through a raw file descriptor. Only AsyncIoStreams are allowed now.
• Moved the tests to use asyncio, and removed any tests and examples that were specifically concerned with synchronous mode.
• Fixed a bunch of failing tests. Everything seems to run more stable now.
• Removed a lot of dead code.
• Some miscellaneous fixes.

[LasseBlaauwbroek]

Note that I recommend reviewing commit-by-commit. They should be reasonably self-contained.

[LasseBlaauwbroek]

Additional cleanups:

• Deleted test_capability_old.py. It's tests also exist in test_capability.py, except that the old version uses explicit .capnp file loading through a fixture. But this is already tested in load_test.py.
• Remove the .capnp file loading fixture in test_capability_context.py in favor of the approach in test_capapbility.py.
• Remove the option of creating a capability server through the _new_server option. (This seems very ugly, and having two ways to do the same thing means more code-paths to test.)

[haata]

Thanks!