Ensure max packet size is respected  #7

Originally by @pushrax:

The Go library will return an error (ErrPktTooLarge) if a packet is written that exceeds what it thinks is the max packet size.

By default that's 4 MiB. The default values are set in NewConfig(). However, we're not calling NewConfig(), we're building the config object ourselves. This means that in this code MaxAllowedPacket is 0, and it loads the limit from the server. I tested this locally and it indeed is always loading the limit from the server. Thus, at the moment, exceeding the real max packet size will always return a nice Go error.