Surfaced in GRPCCall+Tests.h

Add GRPCHost to store channel config, and to create channels on demand
with that config. GRPCChannels and configs are cached together.

GRPCSecureChannel is now initialized with (nullable) path to a
certificates file and (nullable) name override.

The same mechanism will be used for creating insecure channels, removing
the ability to do it by specifying the HTTP scheme in the address (which
was deemed too subtle for its implications).

Reorganize src/python.

Run test server locally

Regenerating project files and massaging VS project files.

This is in preparation of moving all tests into a separate package to
enable ease of coverage checking and testing.

Move grpc_census_call_*_context calls into public header

Python documentation generation.

Reconnect with jitter.

Make ruby tests see changes to C core

Update interop test descriptions.

Reconnect interop spec

make accessors from get_x() to x()