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
Merge remote-tracking branch 'origin/master' into NEXUS-6220-conn-reuse
Merge pull request #315 from sonatype/NEXUS-6220-conn-reuse

NEXUS-6220: Be smarter about connection reuse