PR with rebase to detect first rc_override

Hi Tridge,

Alas - so far I'm unable to repro any failure to connect.  It seems like the old 1.7 code, the beta 1.8 and this new code all seem to loose sync occasionally.  (leading to occasional bursts of 'bad' in the test tool.

With wiprebase:
Veh:220541/32324/6642  GCS:6642/190868/184246  pend:36295 rates:43/2000 lat:30/127/78 bad:0/445 txbuf:87/100
Veh:220592/32334/6644  GCS:6644/190922/184298  pend:36294 rates:43/1500 lat:30/127/78 bad:0/445 txbuf:85/98

with tridgewip (the 1.8 release):
Veh:52513/7710/1580  GCS:1580/45640/44062  pend:8451 rates:43/2000 lat:36/123/75 bad:0/75 txbuf:88/100
Veh:52581/7720/1582  GCS:1582/45697/44117  pend:8464 rates:43/2000 lat:36/123/75 bad:0/75 txbuf:90/100
Veh:52649/7730/1584  GCS:1584/45753/44171  pend:8478 rates:43/2000 lat:36/123/75 bad:0/75 txbuf:94/100
Veh:107967/15812/3252  GCS:3252/93894/90648  pend:17319 rates:43/2000 lat:26/123/76 bad:0/157 txbuf:92/100
Veh:108035/15820/3254  GCS:3254/93950/90702  pend:17333 rates:43/2000 lat:26/123/76 bad:0/157 txbuf:94/100