Add the ability to resume partially failed syncs from head

[masih]

Syncing a chain successfully fires a SyncFinished event that contains the complete list of all the CIDs downloaded. But if the sync fails mid-way through, then an error is returned with no information about the CIDs synced so far.

When syncing extremely long chains from head that consistently fail mid-way, e.g. after hitting some limit on publisher's side, the user of go-legs has no choice but to retry the same sync again, which will never complete.

Add the ability to resume a partially failed sync in legs. Potential approach: return retry-able error that carries enough information for go-legs to re-populate the syncedCids slice and pick up the sync just after where the error occurred.