[#12048] Add verification migration script

[NicolasCwy]

part of #12048

Outline of Solution

• Separate migration and verification script
• Fix progress of verification check (Was floored to 0 before multiplying by 100)

[ziqing26]

Let's rename MigrateAndVerify script to Migrate only and remove verification script from it?

Btw UsageStats check might fail currently, because new stats are generated since then.

[NicolasCwy]

@ziqing26 The row count script will fail but since the verification scripts are using the migrated Postgresql entity to query datastore, the usage statistic script shouldn't fail even with new statistic objects.

The row count script will write that not all datastore usage statistics are migrated though

[ziqing26]

LGTM!