Support cooldowns

[td202]

Purpose

Closes #101

Support protocol version 7, and in particular cooldowns on accounts.

Changes

• Add endpoint /v1/accBalance that exposes the cooldowns on the account (accountCooldowns) and the available account balance (accountAtDisposal).
• Revise /v0/bakerPool so that pools that have been closed are treated as not found. (Note: from protocol version 7, a pool can be closed but still participating in consensus and earning rewards for a time. Such pools cannot accept delegation, and no longer have the full pool details such as metadata URL etc.)

Checklist

• [x] My code follows the style of this project.
• [x] The code compiles without warnings.
• [x] I have performed a self-review of the changes.
• [x] I have documented my code, in particular the intent of the hard-to-understand areas.
• [x] (If necessary) I have updated the CHANGELOG.

[Radiokot]

Agreed to remove currentBalance from the v1 balance endpoint, as it seems useless in wallets