UniqueConnec should allow dialing again if errored

[tomaka]

What should happen if we call UniqueConnec::get_or_dial, but the dialing fails with an error, and then we call get_or_dial again?
Should it immediately return a future with the error, or should it discard the error and try dialing again?

I can imagine that both behaviours are valid.

[tomaka]

Right now we immediately return a future with the error, but in the situation where the error is a timeout we probably want to dial again.

[tomaka]

Done in #446