Fix missing block number issue on forced canonicalization (#12949)

* Fix missing block number issue on forced canonicalization

There is this issue about missing block numbers on forced canonicalization. I looked over the code
now 10000 times and there are possible ways this can be triggered, but I don't really know how this
is triggered. So, this pr is going to solve the symptom and not the cause. The block number to hash
mapping is set when we import a new best block. Forced canonicalization will now stop at the best
block and it will canonicalize the other blocks later when the best block moved. As the error
reports indicated that this issue mainly happened on major sync, there should not be any forks, so
not doing the canonicalization directly shouldn't be that harmful. All known implementations should
import all blocks as best block on major sync anyway (I mean somewhere there is the bug, but I
didn't yet found it).

I will also do some changes to Cumulus around some potential culprit for this issue.

Closes: https://github.com/paritytech/substrate/issues/12613

* Add some docs

* Fix fix

* Review comments

* Review comments