If all peers have been tried during the block download process and some
hashes are unfetchable (available peers > 0 and fetching == 0) throw an
error so the process can be aborted.
* Downloader's peers keeps track of peer's previously requested hashes
so that we don't have to re-request
* Changed `AddBlock` to be fully synchronous
Properly ignore blocks coming from peers not in our peer list (blocked)
and do never request anything from bad peers. Added some checks to
account for blocks known when requesting hashes (missing parents).
Properly ignore blocks coming from peers not in our peer list (blocked)
and do never request anything from bad peers. Added some checks to
account for blocks known when requesting hashes (missing parents).
