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.

If a peer fails to respond (disconnect, etc) during hash downloading
switch to a different peer which has it's current_hash in the queue's
peer set.

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

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