Index FS Store: Allow to cache (in memory) specific files

[kimchy]

When using file system based storage, caching specific (lucene index) files make sense to speed up operations. The ability to specify which files to cache in memory (with all the parameters the memory store provides) based on their suffix should be provided. This allows, for example, to store term freq in memory, while keeping the field store on disk (which takes much more space).

The configuration should look something like this:

index:
    store:
        fs:
            memory:
                enabled: true
                extensions: ["", "del", "gen"]

[kimchy]

Index FS Store: Allow to cache (in memory) specific files, closed by fa55c40