Geo replicates data from primary to secondaries. It verifies data
integrity by checksumming the data on the primary, then
checksumming the data on secondaries and comparing the result.

For performance reasons, on the primary, we need to store the
checksums and other metadata in a separate table from the
source table. For example, the primary checksum data for
`merge_request_diffs` is stored in
`merge_request_diff_details`. The verification logic already
mostly exists. But it depends on the checksum table having a
record when the source table has a record.

This commit adds a worker which inserts and deletes
checksum table records without requiring triggers or the
like on the source table. It is acceptable for this process to
be slow.

This worker is behind a feature flag verification_state_backfill_worker

EE: true