Enabling the search by parent enables the rewrite
of the ListChildProjects REST-API that extracts the children
of a project. Using the Lucene index avoids the full scan
of the ProjectCache and thus speedup the extraction and
reduce the JVM heap consumption.
Bug: Issue 10401
Change-Id: I568d6ba0e5a0bff391f57e2357de6d059d77fe5d
30b0a80290