Instead of asking for a block and reacting when we get a failure, this
just uses the current height instead.
This also makes 'flush' happen in each indexers' own thread, which should
be good for performance.
where the UTXO allows you to find an output based on txid+output, the
spentoutput DB turns that around and allows you to find an input that
spends a certain output.
This also fits perfectly find into the UTXO database class, so this
creates a new dir "spent" which is filled as expected.
tomFlowee
TomZ