rtpmanager: twcc: Fix duplicate packet handling

The previous code would only check if two packets in a row were duplicates. If not (i.e. a packet is a duplicate of a packet received slightly before) the code would generate completely bogus FCI because it assumes there were no duplicates present in the array.

In order to be efficient, just store all received packets and remove the duplicates just before the FCI is generated once the array of observations have been sorted by seqnum.

Fixes TWCC usage with moderate to high packet duplication.