Caching MimeUtil2 responses.

Calling into MimeUtil2 hides a point where
congestion might happen, as all these calls
are hitting a single j.u.Properties instance.

This change adds a cache a top of MimeUtil2
(actually in Nexus component wrapping it),
and should reduce actual calls to MimeUtil2.

The calls to guess MIME types are happening often, at high rate.
It happens at every creation of StorageFileItem.
Removed unused imports.
Applied comments
Applied Ben's comment.
Merge pull request #831 from sonatype/mimeutil2-response-caching

[REVIEW] Caching MimeUtil2 responses