fix(sync): don't lose nodes on single download err

`P2pCandidate.dlPromise` promise doesn't resolve on download errors
such as socket timeouts. Each time such an error occurs, the node is lost
for the sync because the 'this.dlPromise.isResolved()' check used for
testing the node's availability never succeeds anymore for this node.

This patch hardens the blocks downloading in two ways:
* Retrying the download up to 5 times when the number of returned blocks is
  different from what was requested
* On failure returns an empty block array so that the dlPromise is resolved
  and the node failure could be handled as expected at the P2PSyncDownloader
  level triggering the `P2pCandidate.addFailure()` function.