Asynchronous connect for caf-net

[Neverlord]

CAF 0.19 introduced a new high-level DSL for spinning up connections to servers. For example:

  namespace ws = caf::net::web_socket;
  // ...
  auto conn
    = ws::with(sys)
        // Connect to the given URI.
        .connect(server)
        // If we don't succeed at first, try up to 10 times with 1s delay.
        .retry_delay(1s)
        .max_retry_count(9)
        // After connecting, spin up a worker to manage it.
        .start([&sys, hello](auto pull, auto push) {
          // ...
        });

This is fine when doing this from the main thread that has no other task than booting up the workers. However, it's a blocking API and thus should be avoided from cooperatively-scheduled actors.

CAF should offer an asynchronous API alongside the blocking functions, e.g., async_connect, to establish the connection in some background thread.

Currently, we would have to support the new async-connect in these protocols:

• WebSocket
• Length-prefix Framing