Save memory by memory-mapping more files

[jan-niestadt]

Maybe the terms file could be memory-mapped? The format would probably need to change for this.

If we integrate the forward index and content store with Solr, we will probably get this "for free", because the way Lucene deals with segment files seems to include memory-mapping on 64-bit JREs on Linux, see FSDirectory.open() (will use MMapDirectory).