* Updates headers with ASF source files header

* change license to additional files

* 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)

Changes includes:
  * Fixes for 64bit ledger ids
  * Fixed memory leak on read-only bookies
  * Fixed datasketches metric provider concurrency issue

Changes included:
 * [a25b53] Removed unused SortedLruCache
 * [74cd2f] Removed unused DbLedgerStorage configuration keys
 * [fb279e] Reorder read operations based on Bookie availability
 * [d203c2] DataSketches metrics provider
 * [17ab66] Catch IndexOutOfBoundsException when reading entry log index map
 * [270b69] Fixed NPE when accessing readonly bookie list

Changes included: 
 * [c4f1f9a] Avoid RocksDB get() operations during read-ahead cache filling (#145)

Related to #110

 * 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