More filters for crsql_changes

[tantaman]

crsql_changes will behave like a normal table after merging https://github.com/vlcn-io/cr-sqlite/tree/more-filters (eta: 4/24)

• Can use all operations for filtering
• Can filter site_id, db_version, col_version, pk, etc.
• Filters against NULL will behave the same as how SQL treats NULL comparisons