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).

And move it to the RxLibrary.

Part of fixing https://github.com/grpc/grpc/issues/2192

And free ops_array that was leaking.

Introduced grpc_byte_buffer_init instead. It's now the user's responsibility to
manage memory.

In library projects (cf. app projects) the main bundle is nil. `NSBundle+bundleForClass:` works in both types of projects.

Also makes the library load the certificates only once.