NEXUS-5472: Cancelable WIP

Just for backup, WIP
Merge remote-tracking branch 'origin/nexus-5472-proxy404' into nexus-5472-proxy404-cancelable
NEXUS-5472: Cancelling scrapes WIP
NEXUS-5472: Hacking UTs

The OOM on bamboo was caused by old UTs not prepared for async behaviour of
new proxy404 feature. Making the feature pretend dead, but still, some
(newly added) UTs does need it.