Rename NodeStatus to NodeMetadata

Conflicts:
	lib/droonga/engine_state.rb
Use the term "metadata" isntead of "status" for remote commands
Split "role" to two axis: role and type.

old                            => new
role=protocol-adapter          => type=protocol-adapter
role=engine                    => type=engine, role=service-provider
role=engine-absorb-source      => type=engine, role=absorb-source
role=engine-absorb-destination => type=engine, role=absorb-destination
Fix mismatched method name