Complete Message replay immediately if there is no entry to replay which will not block reads from other consumer (#103)

 * 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

In managed ledger BK test, wait for background ledger roll-over to complete before shutting down (#35)

Fixes #16: Intermittent test failure in ManagedLedgerBkTest.testConcurrentMarkDelete

Replace Collections.sort(list) with list.sort() to support earlier jdk-version than jdk-1.8.0_20 (#24)

Fixes #20