Merge results from multiple Lucene subindexes

Instead of building a union of the results, merge the results.  This
allows a future change to support a sorting option, and Lucene's
native merge will perform a merge-sort that selects the top N
documents, ignoring the subindex the document came from.

Change-Id: Iab731bb5979156a6bafeace1cd73d7f24a541227
2 files changed