Manage free downlink HARQ processes with a circular list.

The oldest PID (the PID that was used the longest
TTI ago) is at the head and is the next one that
will be used.

No test is done to check that when a process is
allocated it is at least N TTIs old, but some quick
thinking strongly leads to the 'conclusion' that N
won't be smaller than 8, which seems to be what
cat3 UEs require to function properly in the downlink
direction (from the eNodeB to the UE).