Rewrite nrepl connection logic

- Extract networking functionality (`nrepl-connect`, `nrepl--direct-connect`,
  `nrepl--ssh-tunnel-connect`)

- When `nrepl-force-ssh-on-remotes` is nil try a direct connection with remote
  hosts before attempting a ssh tunnel.
Yeah, I got the idea right away. I don't have a problem with it in general, just this sentence in particular. The parens variant looks OK to me.