This commit refactors the roaring bitmaps to use uint64 values
instead of uint32 values. This is required for the size of
values we need in pilosa.
This change is not backwards compatible with the previous data
format so any existing data directories need to be removed before
using this code.
This commit adds an implementation of roaring bitmaps that can be
used with mmap files and also allow for delta changes that are
appended to the end of the file as a write-head log.
