Cache scrypt hashes on disk

Cache scrypt hashes on disk

instead of recalculating every time. This policy is able to give us 10-20x startng speed up. User can disable caching with -fastindex=0 option.

Note that this commit makes blockindex code incompatible with old blockchain database files. You need to remove old blockchain files and syncronize with the network again.

Another changes:

1. Stake pooled keys are removed.