Implement MetadataCache for VirtualRepositoryReader

There are several places for metadata in Maven repo:

• local repo
  • GA group level maven-metadata-local.xml
  • GAV level maven-metadata-local.xml
    • maven-metadata-local.xml
    • resolver-status.properties
    • maven-metadata-forge.xml
    • GAV.jar.lastUpdated
    • GAV.pom.lastUpdated
• remote repo
  • GA group level maven-metadata.xml
  • GAV level maven-metadata.xml

Problems

• I don't see any difference between maven-metadata.xml and maven-metadata-local.xml
• we don't need this metadata.xml in local repo - it's more reliable to scan directories than keep directories and this file in sync
• this metadata.xml is only needed for "dumb" remote repo to prevent page scraping
• there no standard place to record remote repo scan timestamps, each component seem to deal with it on it's own

Solution

• does not exist yet - needs to be designed
• should be compatible with at least existing metadata.xml files
• should allow standard recording of repo scan timestamps
• should be flexible to allow recording of other GA and GAV attributes