Added a cancel method to the downloader which gracefully shuts down any
active syncing process (hash fetching or block downloading) and resets
the queue and remove any pending blocks.
Issue with the downloader which would stall because of an active ongoing
process when an invalid block was found.
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
