Write the precedent bootstrap step on the disk

[AurelienFT]

Context

In the bootstrap process the ledger is splitted in part and streamed between the bootstrap server and client. It can be splitted at an arbitrary step at a entry of the ledger if the entry is two big.

Problem

Currently, if the client is disconnect the step where the bootstrap stopped is sent to the next node we bootstrap on. If we stop the node this information is lost. To save it we need to store it on the disk.