Allow starting syncer with head that is not recent

[gupadhyaya]

In case of single node (sequencer) with very short blocktime, after the genesis block is published, it tries to start the syncer, but fails to use the latest head as it is not recent enough (https://github.com/celestiaorg/go-header/blob/main/sync/sync_head.go#L24). It cannot fetch from the network as there are no peers. In this situation, can we allow starting syncer with not recent enough head?

This scenario is happening in rollkit with sub-second blocktime, where looks like the time between the genesis block timestamp and when the node tries to start the syncer is larger than blocktime*blocktime/2 as allowed in isRecent.

[Wondertan]

If we fail to fetch getter.Head, we can return s.subjectiveHead, while logging out the error.

This will solve your problem and I can definitely say that's a nice improvement to the syncer, I. E. syncer continues functioning even when exchange fails. This is fine as long as our subjective is not expired.

[renaynay]

also good w me