Unify LiveNodesList to ClusterState

Conflicts:
	lib/droonga/cluster_state.rb
	lib/droonga/engine_state.rb
Use the term "active_nodes" instead of "live_nodes".

The "live_nodes_list" has been unified to the cluster state.
Load required module Path
Fix mismatched variable name
Simplify contents of the file cluster-state.json
Output "info" log when cluster state is reloaded, even if the state is not changed