dccp tfrc: Receiver history initialisation routine

This patch
 1) separates history allocation and initialisation, to facilitate early
    loss detection (implemented by a subsequent patch);

 2) removes duplication by using the existing tfrc_rx_hist_purge() if the
    allocation fails. This is now possible, since the initialisation routine
 3) zeroes out the entire history before using it.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>