NewTargetTracker returns a new target queue.

BadTipSetCache keeps track of bad tipsets that the syncer should not try to download.

Target tracks a logical request of the syncing subsystem to run a syncing job against given inputs.

TargetTracker orders dispatcher syncRequests by the underlying `TargetBuckets`'s prioritization policy.

just compatible code lotus.

TargetBuckets orders targets by a policy.