Configurable bitmap indexes: roaring and concise

Do not merge this pull request yet! More tests are required. All original unit tests pass at this point. I only want to start a discussion on high level changes. This also requires the 'fixup' branch in bytebuffer-collections

@lemire @samytto - we require some more support for complementing in Roaring
@cheddar - if you only want to look at one thing, the changes to the queryable index interface are what bothers me the most
@xvrl @gianm @nishantmonu51 @drcrallen - would love to hear comments on the high level changes
 
let's document the entire index.drd format while we're at it