Un-subscribe all gossip when reverting to finalized sync

[dapplion]

Currently we un-subscribe core topics when going to finalized sync

https://github.com/ChainSafe/lodestar/blob/2ed2e03d0ca3e2d6e04e503d4e1beb2deedd47a7/packages/lodestar/src/sync/sync.ts#L193-L194

However we keep subnet subscriptions, which are actually the ones taking the bulk of CPU processing.

• [ ] Un-subscribe all gossip when reverting to finalized sync

From https://github.com/ChainSafe/lodestar/issues/2488

[philknows]

Please investigate @tuyennhv , this may already be completed.

[wemeetagain]

This has not been completed, still a real feature request

[twoeths]

will look into this