Add optional column for `schema_version` in `crsql_changes`

[tantaman]

This will allow us to ensure, at the library level, that incompatible schemas are not synced.

[Azarattum]

Is this supposed to be an integer that is incremented with every migration or some kind of hash from the actual table structure? I think the later would be more robust, since it would prevent an accidental sync between completely different DBs.