merge databases

Merge multiple databases together. This should work without requiring the presence of the raw data so that the results from multiple computers can be combined together without transferring the data file (big), only the tree data (small) plus ranges for the data blocks (unknown size, probably smaller than big but bigger than small).

Merging databases works in the merge example but the ergonomics is not yet very good. Leaving this issue open until there is a top-level API method for doing merges instead of parsing the range file. The top-level merge API should also be flexible in how the pointers to the other data files are encoded.