Add a LedgerMaster function to get a ledger's
close time from either its hash or sequence number.
Use this function when adding the 'date' fields to
transaction JSON. This avoids constructing large numbers
of ledgers.
* Move InboundTransactions to app/ledger
* Move TransactionAcquire to app/ledger
* Move LocalTxs to app/ledger
* Move Transaction to app/misc
* Move TransactionMaster to app/ledger