Queries for accounts with scheduled releases or cooldowns

[td202]

Purpose

Closes: #1263
Depends on: Concordium/concordium-grpc-api#67 Concordium/concordium-base#575

Add gRPC endpoints for getting the lists of accounts with scheduled releases or cooldowns.

Changes

• GetScheduledReleaseAccounts endpoint
• GetCooldownAccounts endpoint
• GetPreCooldownAccounts endpoint
• GetPrePreCooldownAccounts endpoint

Checklist

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