BlockHoover: track remaining blocks.
use enum for OP_XXX, new fun is_unspendable().
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.
mods for new utxo scheme.

cache_version: 3
feed_tx: pulled out of feed_block.
catch_up: re-enable latency warnings.
make PUSH_OP etc available from python.
when synchronizing, show count of remaining blocks.
new utxo.
new utxo.