[Clearinghouse] Track splits of combined donations

Clearinghouse money comes in a one lump sum. We need to track how they're split (all the individual disbursmenets):
- Know where the money went
- Understand what money hasn't been sent out here
- To allow for an "unclaimed funds" policy.

No files, tests, or entry points are mentioned. Start by tracing how Clearinghouse lump sums and individual disbursements are represented, then define how splits and remaining funds should be recorded; done means each combined donation's destinations and undistributed balance can be identified for an unclaimed-funds policy.

Written by the indexing model from the issue text.