* Upgrade to netty-4.0.46

* Fixed DoubleByteBuf leak detector

* Exclude netty-common from datasketches-metrics dependency

* Upgrade async-http-client to latest stable version (fix incompatibility with netty)

 * Changed Guava cache with Caffeine async loading cache
 * Added async lookup methods in ZooKeeperDataCache
 * Changed namespace bundle ownership registration to be asynchronous
 * Use Optional<> to treat ZK NoNodeError instead of propagating the exception to the cache
 * Changed HTTP topic lookup to use async servlet
 * Fixed MockZookeeper to notify watchers without holding instance mutex to avoid deadlocks in unit tests