* Allow for delta tables to be directly backed by storage.
Enables experimental support for delta tables that are too large to be
stored in memory. We directly expose `DeltaTable` instead of copying the
data into a datafusion::Memtable.
Disadvantages:
- in the new mode, no support for S3
- as we're relying on datafusion to handle the parquet files directly,
nested schemas and certain data types may not work properly.
