"Closable" HTTP client instance

In some cases, you need to be able to clean up all resources allocated by the HTTP client: close all open connections, shutdown appropriate threads etc. It's useful and desirable e.g. when your application has some kind of lifecycle management or when implementing a library that targets to be a good citizen for your JVM.

All of that is kinda tricky with Aleph, as we do not have any specific notion of "client instance" at all, at least in a way we do for the server. By default, HTTP request goes to a default shared connections pool and operates on shared Netty's threads group. But we can expose another API that creates a reified `Closable` instance that you can use to call `.close` when necessary (like we do to shut down a server), uses provided instance of a connection pool or newly created one (but never shared), spawns separate Netty's group to shutdown it when necessary.

Start by reading Aleph's existing HTTP client behavior, shared connection-pool handling, and server shutdown API; the issue names no files or tests. Done means a reified client instance can close its own connections and dedicated Netty threads without affecting shared resources.

Written by the indexing model from the issue text.