Below is the cleanup process after Cloud gets a commit decision for a tenant migration:

1. Update DNS to forward all client requests to the recipient.
2. Send donorForgetMigration to the donor.
3. Clean up tenant databases on the donor a short grace period of 15 minutes after step 1. This is safe since all cursors have the default timeout of 10 minutes and noCursorTimeout is blocked on the proxy.

For step 3 to work (i.e. not fail with TenantMigrationComitted), tenantMigrationGarbageCollectionDelayMS must align with the grade period. So we should decrease its default value from 48 hours to 15 minutes.