Do blocking only for migrated slots during migration finalization

For now when we do slot migration finalization we block all requests for all slots, using standard PAUSE mechanism until the end of the finalization process. We can improve user experience if we block only requests to the migrated slots.

Locate the slot migration finalization path and the standard PAUSE mechanism, then trace how requests are currently blocked until finalization ends. Determine where migrated slots are identified and verify the behavior for migrated and non-migrated slots. Done means only requests targeting migrated slots are blocked during finalization, while other slots remain available.

Written by the indexing model from the issue text.