improve vote version/votebits handling

[jolan]

A user can upgrade before the pool or the pool can upgrade before a user. We need to handle these intervals better.

dcrstakepool:

• [ ] expand api to report server vote version and accept the client's vote version
• [ ] report vote version on home page or status page
• [ ] migrate Users.VoteBits Users.VoteBitsVersion to UserVotingPrefs table and allow votebits to be set for current vote version (N) and N+1.
• [ ] validate stored N+1 votebits on upgrade
• [ ] when client vote version > server vote version = record vote preferences for when upgrade is performed? report success but still warn and tell the user to nag the admin to upgrade?
• [ ] when server version > client version = error out and warn that it's time to upgrade?

decrediton/paymetheus:

• [ ] pass vote version
• [ ] test that errors/warnings are displayed in a clear, understandable manner

[jolan]

I don't think I will have time to work on this any time soon. This should be implemented when the next mainnet voting happens though.