approval-distribution: aggresion must target unfinalized chain rather than unapproved chain (#2988)

Found the issue while investigating the recent finality stall on Westend
after upgrading to 1.6.0. Approval distribution aggression is supposed
to trade off bandwidth and re-send assignemnts/approvals until enough
approvals are be received by at least 2/3 validators. This is supposed
to be a catch all mechanism when network connectivity goes south or many
validators reboot at the same time.

This fix ensures that we always resend approvals starting with the first
unfinalized block even in the case when it appears approved from the
node's perspective.

TODO:
- [x] Versi test

---------

Signed-off-by: Andrei Sandu <[email protected]>