Master lease

I think this works as intended (see http://jira.incubaid.com/browse/ARAKOON-468 ) .. but I want some comments about how to organize it.

I'm not really satisfied with the RenewLease module in node_main which allows creating a few `unit Lwt.t`s which should then be ran.
I was thinking maybe these loops should be written as 'actors' ?
(I could add a few helpers which allow easily defining a few actors)
Instead of returning the unit Lwt.t these could be `Lwt.ignore_result`ed, maybe that's cleaner?

I could move the RenewLease module to another file (probably somewhere in the /paxos dir).

Feedback wanted.