* Allow customizable ZK update conditions

* Add copyright

* Add unit test, put update condition directly into load manager

* handle zkCache failure: invalidate cache and zk-getData failure

* introduce separate executor to serve zkcache callback

* add executor to discovery service

* remove testing npe

* add assetion on zkCache executor parameter and update tests

* update executor thread in testcase for intermittent failure

* Remove exceptionally completed topic futures

* Ensure more recently created futures are not removed

BrokerService should not remove failed topic from map with the same thread which adds in map to prevent deadlock (#375)

* Avoid using zk-client thread to process zk-data result

* add ThreadPool-name and stop executors on shutdown

* shutdown zkCache executor if zkCache has initialized it

* Fix LoadManager shutdown timing issue

* Remove synchronization on close()

* Make candidate selection only consider active brokers

* Fix bugs introduced by new load manager

* BrokerConsumerStats - refactored the server side logic to get stats.

* Addressed Andrew's CR comments

* Addressed Matteo's and Rajan's CR comments

* Addressed Matteo's and Rajan's CR comments

* Corrected comments in proto file

* Changed field number to 4