NX-684: Registry root expiration fixes

Multiple problems fixes and improvements added:
* NX Core handled NPM MD as expired (as it sees it as coming from localstorage/cache)
* NPM Proxy MD service had a bug, that considered NPM reg root always as expired as it was incomplete
* improved logging with some TRACE logs
* added IT ensuring that once cached, registry root is not re-fetched

Issue
https://issues.sonatype.org/browse/NX-684
True, fixing it. The lock existing in proxy code doing the call into proxy MD service is shared, and hence, concurrently entering threads are possible.