Queries for accounts with scheduled releases or cooldowns.

[td202]

Purpose

Issue: Concordium/concordium-node#1263
Related: Concordium/concordium-grpc-api#67 Concordium/concordium-node#1260

Support a GRPC endpoints for getting the list of accounts with pending cooldowns or scheduled releases.

Changes

• Update GRPC
• AccountPending type for indicating that an account with a first pending release or cooldown at a particular time.

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.