simplify utxo map design.
simplify utxo map design.

The previous design caused multi-second hangs when updating outpoints
containing thousands of outputs.  Instead of a two-stage data
structure, we flatten into a single tree, with one output per node.

This requires a change to the utxo.bin file format, hence a bump in the version number to 3.

samrushing/caesure,samrushing/caesure