NEXUS-7609: Prevent saving attributes

Even if repo expiration happened, and proxyCache token
is set.
NEXUS-7609: Prevent saving attributes, proper fix

0d9e3f4c1faad507c6ab0f96218cfde9b8d75620 change was not thread safe,
and also it was only partially fixing the problem. In general,
NX Core should NEVER handle NPM metadata items as expired/old.
Merge pull request #14 from sonatype/nexus-2.11.x-NEXUS-7609-attributes-saved

NEXUS-7609: Prevent saving attributes