Brute force blockchain tampering via unchecked length field (whose validity is not checked during consensus) ; absent logging

see: https://github.com/dvf/blockchain/issues/50

The underlying exploit to tamper with a blockchain is described there

props to: @TimelessP

The fix should be fairly simple:
affected file: blockchain/blockchain.py

https://github.com/dvf/blockchain/blob/4010cf3273e19146a9cd7b37cf355cb751ffef88/blockchain.py#L82

A noted, currently it reads:

if length > max_length and self.valid_chain(chain):

@TimelessP suggests a change as follows:

if length > max_length and self.valid_chain(chain) and length == len(chain):

(and then of course the same fix needs to be applied to the C# code)

Additionally, be sure to log offending hosts.