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

Reviewers:
  @JoelKatz @donovanhide

This is in response to "blocker" RIPD-698.
Oh, wow. Then the issue is probably just a tiny bug in walkHashBySeq.
https://github.com/ripple/rippled/blob/develop/src/ripple/app/ledger/LedgerMaster.cpp#L1361-L1370

This fails if the ledger you're asking about *is* the last valid ledger.