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.
Seems to me we've forgotten to close the open nREPL client sessions with the `close` op. We're currently leaking nREPL sessions every time we disconnect from the server. A small bug, but something to keep in mind.