Freelist stats

Adding this made me realize that bolt's FreelistAlloc is really FreelistInuse and reflects just the space used by the freelist itself, whereas I'm more interested in actual free page space. Not sure if we want to push some of that clarification back into bolt or not since it's mostly redundant info. But I'd be happy to do it, if you think so.

@benbjohnson 

/cc @snormore, @charlescng 
https://github.com/boltdb/bolt/pull/201