This commit adds two new funtions named ReadMessageN and WriteMessageN
which return an additional paramter for the number of bytes read or written,
respectively.
It also adds tests to ensure the number of bytes read and written are the
expected values both for successful reads/writes and unsuccessful ones.
Closes#6.
This commit changes MsgBlock to enforce a 1MB max payload per the spec.
Previously it was only limited to the max overall message size. While
here, also enforce max payloads per message type (instead of only the max
overall message payload) when writing messages.
