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

Reviewers:
  @JoelKatz @donovanhide

This is in response to "blocker" RIPD-698.
Should the logic not be changed here:
https://github.com/ripple/rippled/blob/develop/src/ripple/rpc/impl/LookupLedger.cpp#L144-L149
So that ledger_hash and ledger_index are always set?