Keep sessions in window for the full unfinalized chain (#6054)

* Impl dynamic window size. Keep sessions for unfinalized chain

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

* feedback

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

* Stretch also in contructor plus  tests

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

* review feedback

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

* fix approval-voting tests

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

* grunting: dispute coordinator tests

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

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