approval-distribution: batched approval/assignment sending (#6401)

* Imple batched send

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* Add batch tests

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* pub MAX_NOTIFICATION_SIZE

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* spell check

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* spellcheck ...

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* Fix comment

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* o.O

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* 2 constants

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* Ensure batch size is at least 1 element

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* feedback impl

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>