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).

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

This reverts commit 6f34bad5.

grpc namespace and not supported by older compilers. Not sure
why I didn't catch this earlier.

Only one of these is used, and in only one place, as of this commit,
but they will be used widely after further development.