Fix approval-voting canonicalize off by one (#6864)

Approval voting canonicalize is off by one that means if we are
finalizing blocks one by one, approval-voting cleans it up every other
block for example:

- With 1, 2, 3, 4, 5, 6 blocks created, the stored range would be
StoredBlockRange(1,7)
- When block 3 is finalized the canonicalize works and StoredBlockRange
is (4,7)
- When block 4 is finalized the canonicalize exists early because of the
`if range.0 > canon_number` break clause, so blocks are not cleaned up.
- When block 5 is finalized the canonicalize works and StoredBlockRange
becomes (6,7) and both block 4 and 5 are cleaned up.

The consequences of this is that sometimes we keep block entries around
after they are finalized, so at restart we consider this blocks and send
them to approval-distribution.

In most cases this is not a problem, but in the case when finality is
lagging on restart approval-distribution will receive 4 as being the
oldest block it needs to work on, and since BlockFinalized is never
resent for block 4 after restart it won't get the opportunity to clean
that up. Therefore it will end running approval-distribution aggression
on block 4, because that is the oldest block it received from
approval-voting for which it did not see a BlockFinalized signal.

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Signed-off-by: Alexandru Gheorghe <[email protected]>