simple mode for the syncclient: from top, gen only one task.

build new plans from an a bunch of checkpoints, we turn n checkpoints into correspondingarray of n tasks adjactent to each other, the last one of the tasks has end at 0NOTICE: the input map of checkpoints will be clear after execution.

an additional cancel func is provided and should be called before the sync task isstopped.

pruen a plan by the data of ledgers to avoiding overhead traffic (duplicatedtransfer for existed blocks).

this help us to access the stub object from streamstub.