gRFC A8: Client-side keepalive

[Eric Anderson]

Discussion for https://github.com/grpc/proposal/pull/22 to perform HTTP/2 PING-based keepalives, and permitting aggressive-enough settings to be used as a weak form of health checking.

This is actually a really old design and predates the gRFC process, but implementation was delayed and I was slow for making the design in-whole public.