Add response cache to serve get children (2) requests.

ZOOKEEPER-3180 introduces response cache but it only covers getData requests. This JIRA is to extend the response cache based on the infrastructure set up by ZOOKEEPER-3180 to so the response of get children requests can also be served out of cache. Some design decisions:

• Only OpCode.getChildren2 is supported, as OpCode.getChildren does not have associated stats and current cache infra relies on stats to invalidate cache.

• The children list is stored in a separate response cache object so it does not pollute the existing data cache that's serving getData requests, and this separation also allows potential separate tuning of each cache based on workload characteristics.

• As a result of cache object separation, new server metrics is added to measure cache hit / miss for get children requests, that's separated from get data requests.