Pass a non-infinite deadline to grpc_completion_queue_next() to prevent queues from blocking indefinitely in poll().

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).

- Moves them to the root of the repo.
- Makes them pass lint (except for the release tag warning).
- Includes the script to rename time.h and string.h in the spec.
- Fixes the #includes in the wrapper files.