[NEXUS-5797] Add UnusedSnapshotRemoverTask/Descriptor
[NEXUS-5797] Add "useLastRequestedTimestamp" to snapshot removal request
[NEXUS-5797] Calculate of a snapshot should be removed based on item last requested timestamp if useLastRequestedTimestamp is set
[NEXUS-5797] Call snapshot remover using "useLastRequestedTimestamp"
[NEXUS-5797] Add UnusedSnapshotRemoverTaskIT
[NEXUS-5797] Make snapshot removal condition easier to read
Merge pull request #73 from sonatype/NEXUS-5797-remove-unused-snapshots

REVIEW: [NEXUS-5797] Scheduled task to remove old unreleased snapshots