Fix p2p node key persistence

crypto.LoadECDSA has been modified to expect hex data.
The key was being saved as raw bytes, causing the file
to be rewritten on every start.

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

* Improved block propagation by sending blocks only to peers to which, as
  far as we know, the peer does not know about.
* Made sub protocol its own manager
* SubProtocol now contains the p2p.Protocol which is used instead of
  a function-returning-protocol thing.