[Yesterday Pushed]: Caching when using FS base store

Explained here:
http://github.com/elasticsearch/elasticsearch/issues/closed#issue/82. I
really like this feature, as it gives the flexability of storing part of the
index in memory, like the term information file (tis). Think, in database
terms, storing the index in memory while the actual data is stored on file

Note, storing in memory parts (or the whole index, as with the memory store)
has no affect on long term persistency (translate: you will not loose data)
thanks to the gateway (write behind) concept in elasticsearch.