Extend MergerStorage Class to Support Both MongoDB and SQLite

[DylanWelzel]

The MergerStorage class is currently designed for use with MongoDB, utilizing MongoDB-specific features such as bulk writes and handling BulkWriteError for merging documents on duplicate key errors. We need to adapt this class to work with SQLite specifically for use in the BioThings CLI.

This work builds on PR #327, which already tackled ignoring duplicate _id values in SQLite.