NEXUS-6486: Do not create status checker threads non-proxies

Create status checker threads only for proxy repositories.

Ported from PR
https://github.com/sonatype/nexus-oss/pull/428

Issue
https://issues.sonatype.org/browse/NEXUS-6486

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF46