Make default value of rocksdb_wal_recovery_mode compatible with InnoDB

Currently, after a non-graceful crash MyRocks fails to start. This is because the default value of rocksdb_wal_recovery_mode is 1: "if a WAL entry is corrupted, the server fails to start and does not recover from the crash."

However, InnoDB by default discards not yet committed transactions. Therefore it is safe to have option 0: "if the last WAL entry is corrupted, truncate the entry and either start the server normally or refuse to start."

It is also more user-intuitive for those who migrate from InnoDB.