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.
Merge pull request #49 from basho/bugfix/lease-default-config

Change default for lease_duration config

Reviewed-by: jtuple

Backport merge of 1d4f8482c13b58c417310997aa1c7ab794f97622