Enable near realtime search in Lucene without external
dependencies. When RAM NRT is enabled, the implementation adds a
RAMDirectory to IndexWriter. Flushes go to the ramdir unless
there is no available space. Merges are completed in the ram
dir until there is no more available ram.
IW.optimize and IW.commit flush the ramdir to the primary
directory, all other operations try to keep segments in ram
until there is no more space.