Features

Add agrpc::notify_when_done, a function for grpc::ServerContext::AsyncNotifyWhenDone that does not suffer from memory leaks like a agrpc::grpc_initiate version would.
Add agrpc::notify_on_state_change, a function for grpc::ChannelInterface::NotifyOnStateChange.
Deprecate agrpc::grpc_initiate, use the above functions instead.
Add a CompletionQueue-based implementation of grpc::HealthCheckServiceInterface. GRPC's default health check service is implemented with the callback API which significantly reduces the performance of a server that also uses the CompletionQueue API (aka. any functionality of this library).

Chore