Fix Flush to construct a valid config string for compact.
Add a fast path for write batches that are either empty or have a single entry.  The problem is that begin/commit causes cursors to be reset, and in this workload there are quite a few cursors.  Only do that if needed to execute the write batch correctly.

This optimization is currently only implemented in RocksDB -- stock LevelDB has no way to figure out the number of entries in a write batch.