Multiple masters

Running CI a second time just to be sure, but looking good so far.
For clarity, I'd consider writing something like
```ocaml
let stepdown ~sides ~n ~i ~some_descriptive_name =
  let sides = (sides @ [ELog (fun () -> "Stepping down to slave state")]) in (* Assuming `sides` are interpreted in-order *)
  Fsm.return ~sides (Slave_wait_for_accept (n, i, some_descriptive_name))
```
higher up in the module, and then obviously
```ocaml
then stepdown ~sides ~n ~i:(Sn.succ i) ~some_descriptive_name:None
```