Added RingBuffer#getall() method

* This call returns a copy of the ring buffer
ChordNode now has a pluggable RTT estimator

* ChordNode.RTTEstimator is used every time, ChordNode has calculated a new RTT
  value. It feeds this value into the RTT estimator for further processing.
* ChordNode.RTTEstimator.rto() is called every time, ChordNode sends out a data
  packet.
* The default estimator can be overriden at runtime by setting
  ChordNode.RTTEstimator from client code.