Set version to 0.18.0
If the RocksDB base file size is changed, change the write cache and L0 size to match
Make --import work
Set version to 0.19.0
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.
Return an empty set if token is not an object
A better way to return an empty set if the token isn't an object.
Set version to 0.19.1
Merge branch 'release' into develop