multi: Implement DCP0013 max treasury spend vote.

[davecgh]

This requires #3548.

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This implements the agenda for voting on the max treasury expenditure policy update defined in DCP0013 along with consensus tests.

In particular, once the vote has passed and is active, the treasury expenditure policy will allow a max of 4% of the balance to be spent per policy window with a minimum floor imposed.

The following is an overview of the changes:

• Generate new version blocks and reject old version blocks after a super majority has been reached
  • New block version on mainnet is version 11
  • New block version on testnet is version 12
• Introduce a convenience function for determining if the vote passed and is now active
• Modify the treasury expenditure policy to enforce the new semantics in accordance with the state of the vote
• Add tests for determining if the agenda is active for both mainnet and the regression test network
• Add tests to ensure proper behavior for the modified policy once the agenda is active as follows:
  • Treasury spends that spend more than 4% of treasury balance when it is more than the minimum floor are rejected
  • Treasury spends that spend exactly 4% of treasury balance when it is more than the minimum floor are accepted
  • The maximum expenditure amount for the policy window is the same regardless of where the treasury spends happen in the policy window
  • Repeat all of the above cases when the treasury balance is low enough that the minimum floor is in effect

[davecgh]

Updated to improve some comments.

[jholdstock]

Looking great, only spotted some minor nits