Upgrade path from single-partition to multi-partition mode

As suggested in debezium-design-documents#1 (comment), the SQL Server connector running in the multi-partition mode might provide an upgrade path from the single-partition mode.

If the connector starts and doesn't find the stored offset for any of its processed partitions, it may try to read the offsets produced by an older version in the single-partition mode.

TODO: specify more detailed requirements:

1. Should this behavior be disabled by default and be enabled via the configuration?
2. If there is an additional configuration parameter, when should it be introduced and deprecated?
3. Should the connector enable the fallback if it processes a single database in the multi-partition mode?