Improve DSN sync to handle continued syncing from non-canonical chain

[nazar-pc]

It might happen such that node has one or more blocks that are not on canonical chain, but the rest of the blocks correspond to canonical chain.

In this case since DSN sync is only concerned with block numbers, it'll be unable to continue sync since parent block will not import. Essentially similar issue to paritytech/polkadot-sdk#493 (at least on high level).

We need to be able to kick in an alternative sub-mechanism that tries to download segments from a bit deeper history and import missing blocks from canonical chain if possible before resuming to sync from DSN the way we do it right now.