Index scheduled transfer events.

[td202]

Purpose

Relates to: #246

This implements indexing for scheduled transfers. The actual queries are not yet implemented.

Changes

• Add scheduled_releases table.
• Add index by (account_index, release_time) for efficiently querying the pending releases on a particular account.
• Add handling for TransferredWithSchedule and TransferredWithScheduleAndMemo events to index the scheduled releases appropriately.

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.

[limemloh]

Looks good to me, but we need some logic for these scheduled_releases to be resolved and update the account balances accordingly.
We can introduce that in a separate PR.

[DOBEN]

LGTM

[lassemand]

Besides

#324 (comment)

it looks good to me.