Move C-layer call creation into GRPCChannel, so that it always acts on a
non-destroyed channel.

Listen for connectivity in the GRPCCall, and destroy the channel.

TODO:
- Documentation
- Make public
- Check I’ve implemented all NSMutDict required methods

So we can make the property a dictionary later, by just extending
NSMutableDictionary to conform to the protocol.

Also clarify in GRXWriter.h that the writeable is released whenever the
writer finishes.

And move it to the RxLibrary.

Fixes https://github.com/grpc/grpc/issues/2194 .