Add a "validated" entry in the full Ledger's Json.

Reviewers:
  @JoelKatz @donovanhide

This is in response to "blocker" RIPD-698.
As explained to me, we can't expect the LedgerMaster to update every single Ledger every time every time there's a status change.

In this one case, we need to make sure we are right up to the millisecond on validation, so the user of the API calls the validation routine.