This change drops the various arguments that force the user to select if
an argument is an .info file or a .db file etc. Instead auto-detect them
and also allow the user to provide a list of arguments.
The most direct effect is that I can now run 'unspent-db info *.info"
When pruning we sort leafs by txid / output and refrain from
writing the txid repeatedly for outputs of the same tx.
Additionally, use the fact that we only ever get to a leaf via
a bucket and since the first 64 bits of a txid is there, skip
repeating them when writing to disk.
Last, make pruning have different strategies.
This should shrink the utxo by about 40%
TomZ