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

Reviewers:
  @JoelKatz @donovanhide

This is in response to "blocker" RIPD-698.
The test should be:

1) Is the ledger already known validated? If so, stop. It's validated.
2) Is the ledger open? If so, stop. It's not validated.
3) Is the ledger's sequence number greater than that of the last validated ledger? If so, stop, it's not validated.
4) Check the ledger's hash against the return value of LedegrMaster::getHashBySeq for this sequence number. If they match, the ledger is validated.