Add grpc-health service implementation

[Scottmitch]

Motivation:
The gRPC community has defined a health checking standard [1]
and API [2]. ServiceTalk should provide a default implementation
of this service.

[1] https://github.com/grpc/grpc/blob/master/doc/health-checking.md
[2] https://github.com/grpc/grpc-proto/blob/master/grpc/health/v1/health.proto

[Scottmitch]

build failure attributed to #1915

[idelpivnitskiy]

Here is another problem now. If you do not clear the map on terminated, clearing status after termination will invoke processor.onComplete() twice. Our PublisherProcessor has protection agains that, but still not ideal to violate the spec here. If the PublisherProcessor impl changes later (or if we let users supply their own processor), it may cause an issue.
I don't think I'm 100% agreed with grpc-java impl to make this method noop after termination. It can still be useful to let users clear NOT_SERVING status.

[Scottmitch]

Yes our PublisherProcessor already protects against this. We could add additional protection here but it currently isn't necessary and I would prefer to avoid adding complexity here. Let see start by adding comments and changing the method name.