The downloader is no longer responsible for processing blocks. The
eth-protocol handler now takes care of this instead.

Added graceful shutdown during block processing. Closes #846

* 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

When a parent is missing in the block list an attempt should be made to
fetch the missing parent and grandparents.