syncmanager: pick the first candidate then break

[detailyang]

Hello.
It looks like we forget to break the loop after picking the first available candidate at syncmanager

[wolfstudy]

please review

if peer.LastBlock() < best.Height {
 	state.syncCandidate = false	
 	continue	 	
}	

if break , next loop the behavior may be different

[detailyang]

@wolfstudy thanks for pointing out. you are right and picking the "first" available candidate to mislead me. It should be the "last" :)