Revert "added locking on _singleSenderStats; untested!"

This reverts commit c542da97076ba0d27f4e191ae5d8249dbf8afe7e.
Revert "added code for AC to send nack packets; no locking yet"

This reverts commit 15bd0878c407ede95f724009adadc81deeb7b4fd.
rollovers are now handled in SingleSenderStats::trackInboundPacket