[NEXUS-5405] Automatically merge group level metadata when "/repodata/repomd.xml" is (re)retrieved from proxy

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5405] JavaDoc

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #128 from sonatype/NEXUS-5405-merge-when-proxy-metadata-changes

REVIEW: [NEXUS-5405] Automatically merge group level metadata when "/repodata/repomd.xml" is (re)retrieved from proxy