REVIEW [NX-525] shared database support

https://issues.sonatype.org/browse/NX-525

also addresses:
https://issues.sonatype.org/browse/NEXUS-6698
https://issues.sonatype.org/browse/NEXUS-6619
https://issues.sonatype.org/browse/NX-601
Ahh, that makes perfect sense. Thanks @mcculls