allow the baker to configure governance flags per address

Baker daemon can bake for several addresses, but governance flags are global per baker daemon. I should be able to bake for several addresses emitting different signals, with a votefile that specifies flags per baker.

This can be done with a votefile formatted as follows:

{
  "tz1xxx": {
    "liquidity_baking_toggle_vote": "pass"
  }
  "liquidity_baking_toggle_vote": "on"
}

Also discussed in:

!4201 (comment 821104760)

@rafoo_ FYI