[gw-v2.0] Timetag logic -- change FIFO memory for ring buffer
The FIFO memory in the timetag logic does not store the latest pulses if it gets full and is not read for a long time. It should therefore be changed for a ring buffer, which overwrites old timetags with new ones and can indeed show when the latest pulses arrive.
This will be done for gateware v2.1.