Fix #91 - Intermittent test failure on SimpleProducerConsumerTest.testSharedConsumerAckDifferentConsumer (#92)

 * 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

Fixes #77

* Use thread safe datetime formatter

This closes #62

* Organize import lines

Add unack-message threshold to restrict consumer for receiving messages without acknowledging-msg up to the threshold (#48)