Prefer fetching small PoVs from backing group (#7173)

* impl QueryChunkSize

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

* QueryChunkSize message

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

* enable fetching from backing group for small pov

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

* review feedback

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

* Refactor `bypass_availability_store`

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

* review feedback

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

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