Change default for lease_duration config
Change default for lease_duration config

Change the default to be `ensemble_tick * 3 div 2` instead of `ensemble_tick *
2 div 3`. The latter always results in a value smaller than the tick
timeout which would result in ticks firing after leases expired. Fix
this so that we properly use leases and maintain safety.

ricardobcl/riak_ensemble,cmeiklejohn/riak_ensemble,cmeiklejohn/riak_ensemble,basho/riak_ensemble,ricardobcl/riak_ensemble,basho/riak_ensemble,sargun/riak_ensemble,sargun/riak_ensemble