Add LedgerCleaner:

* Manually invoked to clean ledgers
* Get status with: 'print app.ledgercleaner.*'
* Invoke or control with 'ledger_cleaner' command
Add BEAST_SQLITE_FORCE_NDEBUG option
Force NDEBUG in sqlite, RocksDB, and HyperLevelDB by default
Begin deprecating the old account_tx interface.