implement aggregator module (#46)

implement aggregator module (#46)

* implement aggregator module
* add DefaultAggregator.newValue()
* integrate aggregator into master-service and worker-service
* ensure worker init() executed after master
* add rpc interface registeredAggregators()
* also add two-tier structure to store current-step and previous-step aggregators
* add repair() for Aggregator
* let master context use config of outer class
* support register aggregator with default value
* add Manager.inited()
* remove worker_id option and use server_port instead
* use rpc-url from master-info