Seamlessly upgrade and migration `schema_info` from pre-4.0.0 to new schema.

[jdaigle]

The schema_info table is completely redesigned for as of 4.0.0. This renders horton.exe incompatible with existing schema_info tables.

It likely possible to detect the old schema and seamlessly upgrade. Perhaps following this general flow:

1. Rename old table. Create new table.
2. For each row in old table, compare using old hashing algorithm to files on disk.
3. If old hash matches file on disk, recompute hash and insert.
4. If old hash does not mach file on disk, or the file does not exist, insert a empty hash. This will force horton to correctly detect changes.