Improve sync start speed on restart

[nazar-pc]

Couple important improvements here.

First of all, instead of starting with last finalized block it makes more sense to start with best block minus archiving confirmation depth. The reason for this is that finalized block is very deep due to paritytech/polkadot-sdk#1570 and we will end up downloading much more data and checking many more blocks than we actually need.

Second tweak is a fix for checking against last processed block rather than finalized block in actual block import that will make things both faster and remove unfortunate situation where last processed block number actually goes DOWN instead of up.

Both of these improvements greatly improve DSN sync, especially on restart (but also after temporary network outages).

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