refactor: Download only non-pruned blocks

[LHerskind]

Altering the L2BlockProposed event and using the lookup allows us to only look if we know there is something to find. To properly test, need to prune parts of the chain to see if we actually download it. This should be possible by extending the e2e_synching test with a chain of length 10 that is pruned back to 5, and then see that after synching we only have the first 5.

Note that we can do this without being able to handle re-orgs it is the fresh sync case from #8457.