dispatcher: don't close UDP connections on failure
dispatcher: don't close UDP connections on failure

just clean the buffer state so we can resume processing new packets

Hopefully this fixes issue #64.

zzl0/carbon-c-relay,lyft/carbon-c-relay,piotr1212/carbon-c-relay,piotr1212/carbon-c-relay,zzl0/carbon-c-relay,rtoma/carbon-c-relay,grobian/carbon-c-relay,tjamesturner/carbon-c-relay,rtoma/carbon-c-relay,Skyscanner/carbon-c-relay,lyft/carbon-c-relay,grobian/carbon-c-relay,lyft/carbon-c-relay,piotr1212/carbon-c-relay,piotr1212/carbon-c-relay,Skyscanner/carbon-c-relay,grobian/carbon-c-relay,tjamesturner/carbon-c-relay