storage_checker: Use '*_get_partition' endpoints instead of querying postgres directly

This avoids violating the storage abstraction, and will allow adding support for Cassandra easily, as the '*_get_partition' endpoints are already implemented for both backends.

It also means switching from ranges of SWHIDs (which are more or less what postgresql uses as primary key) to opaque partitions (because Cassandra's primary keys are based on a different hash that should not be exposed outside swh-storage) like content_get_partition.

Implements the bulk of #4685 (closed)

Depends on swh-storage!1026 (merged) and swh-storage!1034 (merged)