refactoring: connection establishment

[hannesm]

I noticed the current logic, partially in engine.ml, partially in the effectful implementations (app/miragevpn_client_lwt and mirage/miragevpn_mirage), is pretty similar to what happy-eyeballs-{lwt,mirage} are providing.

This package predates happy-eyeballs. I'd be happy to use happy-eyeballs instead of a partial copy thereof.

There are issues with the current implementation (at least in app/miragevpn_client_lwt with a static_client configuration) that lead (for me after a `Ping_timeout (`Restart _)) to a huge number of connection attempts with no delay in between (that's mostly fine); but once one connection is successful there are still ongoing attempts. to the remote (which never end, since the other side is only accepting a single TCP connection).