Add `since` query parameter

[dstotijn]

Add since query parameter to listDiagnosisKeys endpoint to allow offsetting the Diagnosis Keys from the database by created_at to minimize data sent over the wire.

• [x] Add created_at column.
• [x] Add since search support in diag, api and postgres packages.

[ijansch]

The since should not operate on intervalnumber but on submission date. Consider this scenario:

1) On may 1st Alice retrieves keys up until may first. 2) Bob gets infected on may the second and uploads the last 14 days of keys. 3) if Alice now does ?since=20200501 she will miss keys from the period where bob was infected. She needs to be able to match retroactively with bobs history.

[dstotijn]

Yes correct, I should update the description of this issue: I've created a created_at column for this purpose already. Thanks for bringing it up 🙂