Default to not changing dataset schema and definition for SaveMode.Overwrite about filodb

[velvia]

See the details in #72.

A safer default for ingestion is to not change either the existing schema or dataset definition when SaveMode.Overwrite or its equivalent is set. Thus, the default behavior would be that only data is truncated; no new columns can be added, and dataset definition cannot be changed.

Instead we would add two new ingestion options:

• auto_add_columns - defaults to false, but if true, would automatically add missing columns to the table definition;
• reset_dataset - defaults to false, but if true, in overwrite mode would allow the dataset definition - including segment, row, and partition keys, to be reset. Has no effect if the dataset does not exist.