pkg/coveragedb: old data removal w/o transactions

[tarasmadan]

Is your feature request related to a problem? Please describe.
Spanner doesn't support big transactions.
We can't safely add new coverage data and remove all in parallel. As a workaround currently deletion is scheduled in safe time periods.

The alternatives available:

1. Add timestamps to all records and delete only the very old data.
2. Introduce some locking for the DB update/delete operations.

Thanks @a-nogikh for the options.