Switch to new IndexUpdater interfaces
Get index update working with new update interfaces
Update maven indexer version
Updates based on changes to index updater interface
Switch to new pseudo-release of maven indexer

Switch to new pseudo-release of maven indexer, which adds support for
running an incremental-only index update
Merge remote-tracking branch 'origin/master' into NXCM-4867-indexer-sync-simple

Conflicts:
	plugins/indexer/nexus-indexer-lucene-plugin/pom.xml
	plugins/indexer/nexus-indexer-lucene-plugin/src/main/java/org/sonatype/nexus/index/DefaultIndexerManager.java
Change inner class to static

Inner class does not need access to outer class
Update downloadRepositoryIndex to conform to new indexer semantics

The maven indexer semantics were changed to fail if an incremental only
update was requested, but not possible. Previously, it would move on to
a full index update if an incremental failed. downloadRepositoryIndex
needed to be updated to account for this. To keep the behavior the same,
downloadRepositoryIndex will handle a failed incremental only update,
and try a full index update.
Merge remote-tracking branch 'origin/master' into NXCM-4867-indexer-sync-simple
Merge pull request #848 from sonatype/NXCM-4867-indexer-sync-simple

REVIEW: NXCM-4867 - Index update concurrency problem