NEXUS-6220: Expose method returning HC4 that supports keep-alive

To not have to fiddle with getting some RemoteStorageContext
Merge remote-tracking branch 'origin/master' into NEXUS-6220-conn-reuse
NEXUS-6220: Ask do we need connection reuse.

Before it was always "no". But, there are cases where
connection-reuse is needed, according to remote storage context
configuration.
NEXUS-6220: Adjusted Javadoc
NEXUS-6220: UT for reuseConnectionsNeeded method
Change authz handling to allow 403 handling to filter up to jetty so that it can use servlets error-page.

Add a /error.html error-page to split out terse details.
Merge remote-tracking branch 'origin/master' into NEXUS-6220-conn-reuse
Made ErrorPageServlet to serve error pages

And it handle all verbs, not only GETs
Merge pull request #315 from sonatype/NEXUS-6220-conn-reuse

NEXUS-6220: Be smarter about connection reuse
Merge pull request #316 from sonatype/403-error-page

WIP: 403 error page
Merge remote-tracking branch 'origin/master' into rapture
Merge branch 'rapture' of github.com:sonatype/nexus-oss into rapture