Eliminate address reuse

[xaur]

Voting address is currently reused per VSP account. There is no way to change it except starting new VSP account. As an unhappy side effect, it also prevents migration of funds to a new seed.

Assuming roughly half DCR is staked, and roughly half of that uses VSPs, it means roughly 1/4 of holders reuse addresses and have all staked holdings cluster-able.

A workaround is to generate a new VSP accounts, which is problematic (pita) because of another privacy issue - email-based identities, where a new email is needed for a new account (#274).

[dajohi]

Requires dcrwallet changes first - See decred/dcrwallet#1217