BREAKING CHANGE: Stable API for 1.0 release

[standy66]

We need to fix the API for 1.0 release. Unfortunately, there are going to be some breaking changes ;)
Here are some of them:

• stream_stream client call (and maybe stream_unary) should really be an async context manager: there is implicit thread of execution running that's feeding the internal send buffer with data from async iterable. If the exception occurs when sending the data (e.g. server expects 3 messages, but client sends 4), this exception should not be discarded. This is in contrast to Google's official grpc implementation, which in the example above will silently discard the exception. I do not think this is reasonable.
• Give access to request headers and ability to respond with custom headers #4: I think I am going to make optional request argument mandatory context argument.
• Need to expose stream-like API for those, who does not need separate execution thread for stream_stream calls, this may break something, but I am not sure.
• Maybe we should change generated files suffix from _grpc.py to something more self-describing, like _purerpc.py

Overall, I think the API should be as close as it can get to grpcio modulo the corner cases described above.