* Updates headers with ASF source files header

* change license to additional files

* 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

* Recycle EntryImpl of EntryCache

* remove position import from Entry

* Use proper ref-counting semantic for Entry and removed position-impl recycling

* release data after retain into Entry + make variables final in position

* Fixed ref-counting after reading entries

PR from @sschepens  + tests and few fixes from @merlimat

Replaced some usages of AtomicInteger, AtomicLong and AtomicReference
for their Atomic*FieldUpdater counterpart on classes which get
instantiated a lot, this should reduce memory usage for the broker
and clients which use lots of topics and consumers.

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