Preferred master 1.7

* [x] waiting for CI http://172.19.49.85/view/arakoon-git/job/arakoon-git-launcher/358/downstreambuildview/?

After that we probably want to do another backport...
I should add my *monad utils Functor* one day, so this coule be
```
unless skip_prologue 
    (prologue cluster connection) >>= fun () -> ...
```

https://github.com/NicolasT/ocaml-free/blob/d9533271249e19a7c5fee128e53fbbe59f6ceeb8/src/free_monad.ml#L203