[rel-db] Use postgres sequence for sequential_number fields

[peb-adr]

Several collections have sequential_number fields.
These are currently incremented and set in the backend code.

It would be nice to alter the SQL schema such that these values are instead generated/incremented on DB level, most likely using sequences.

We have to ensure that these sequences are initialized correctly (i.e. max(<collection>.sequential_number)). This would be part of the corresponding migration in the backend.