refactor(chain)!: move schemas to their own methods

refactor(chain)!: move schemas to their own methods

We would like to test backward compatibility of new schemas.
To do so, we should be able to apply schemas independently.

Why to change `rusqlite::execute` by `rusqlite::execute_batch`?
- we are going to need to return the statements of the schemas as
  Strings, because we are now returning them from methods, it seemed
  redundant to keep getting references to something is already
  referencing data, i.e., keep moving around &String instead of String
  (consider `rusqlite::execute_batch` takes multiple statements as a
  single String)
- we were calling `rusqlite::execute` with empty params, so we weren't
  trapped by the method's signature
- `rustqlite::execute_batch` does the same than we were doing applying
  statements secuentially in a loop
- the code doesn't lose error expresivity: `rusqlite::execute_batch` is
  going to fail with the same errors `rusqlite::execute` does

BREAKING CHANGE: changes public method `migrate_schema` signature.