Issue #25   Replacing usage of TCP RTT estimator with a customized version

In the current commit, RTT estimator does not implement Karn's algorithm
and takes into account all RTT samples, including ambiguous samples from
retransmitted Interests.
Issue #25: fix the logic in SentSeq and AckSeq to capture retx and support random seq
Refs #25   Merge issue branch 'issue-25-rtt-estimation'