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.
This commit corrects the tests so that the main API functions are tested
against the latest protocol version, but the TxSha and BlockSha functions
are run against the specific protocol version used to encode the test
data. This will help future proof the tests against protocol changes.