Only enable unknown block sync after block is synced

[twoeths]

Describe the bug

As found in #5650 (comment), when node is syncing it may also receive unknown block root event (especially in a setup with fallback urls), there is no need to search for block at that time

also this is to avoid this case

lodestar/packages/beacon-node/src/sync/unknownBlock.ts

Line 228 in 2eddf46

// the common ancestor of the downloading chain and canonical chain should be at least the finalized slot and

Expected behavior

Only start/init unknown block sync once the node is synced

Steps to reproduce

No response

Additional context

No response

Operating system

Linux

Lodestar version or commit hash

v1.9.0-rc.2