DNS: use Netty's asynchronous DNS resolver

The current Netty-based transport doesn't use Netty's asynchronous DNS resolver, which means the client hangs if hostnames cannot be resolved. This is really bad if you are reusing an eventloop from other parts of a program and this eventloop is not supposed to block. gRPC should at least expose the necessary APIs to be able to override the default DNS resolver.

Start by tracing the current Netty-based transport's DNS resolution path and how it uses the event loop. Done means hostname resolution is asynchronous and the necessary APIs allow overriding the default resolver without blocking a reused event loop.

Scritto dal modello di indicizzazione a partire dal testo della issue.