Add Maximum Message Length Check To PeerProtocolCodec

[GGist]

Currently any peers would be able to drain the client machine of memory by sending a message with a large payload (this would only affect variable length message fields).

We should add a max message length to PeerProtocolCodec so that when the codec checks the number of bytes that the message will use, it can kill the connection and propagate an appropriate error (one that clients can identify and filter the peer in the Handshaker).

[GGist]

Not sure if we want to also support the notion of a maximum block size (that validation would probably happen at the PeerWireProtocol layer), or if we want to rely on this global length check for that as well. It could also be something we explore in the future.

[GGist]

Separated block size checks out to separate issue #96