* Renamed ChainManager to BlockChain
* Checkpointing is no longer required and never really properly worked
when the state was corrupted.

I forgot to update one instance of "go-ethereum" in commit 3f047be5.

All code outside of cmd/ is licensed as LGPL. The headers
now reflect this by calling the whole work "the go-ethereum library".

Nodes that are out of sync will queue many transactions, which causes
the initial transactions message to grow very large. Larger transactions
messages can make communication impossible if the message is too big to
send. Big transactions messages also exhaust egress bandwidth, which
degrades other peer connections.

The new approach to combat these issues is to send transactions in
smaller batches. This commit introduces a new goroutine that handles
delivery of all initial transaction transfers. Size-limited packs of
transactions are sent to one peer at a time, conserving precious egress
bandwidth.

to total difficulty

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.