Add vote-validator tool.

[jholdstock]

vote-validator is a tool for VSP admins to verify that their vspd deployment is voting correctly according to user preferences.

What it does

1. Retrieve all voted tickets from the provided vspd database file.
2. Retrieve vote info from dcrdata for every voted ticket.
3. For the n most recently voted tickets, compare the vote choices recorded
   on-chain to the vote choices set by the user.
4. Write details of any discrepancies to a file for further investigation.

[jholdstock]

PR has changed substantially since @dajohi gave his approval above. It's worth another look.