server_queuereader: avoid race on self->failure
server_queuereader: avoid race on self->failure

Don't unset self-failure prematurely, or concurrent writers may get
blocked under false assumptions.  At the same time, report when a server
comes back, so we have a trace of loss + recovery.

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