Port #1218 to 6.x - add optional maximum message size

[lukebakken]

Related to #1213

Port #1218 to 6.x in API-compatible way:

• Version 6.4.0 will be required since public API changes
• Default is to allow unlimited message sizes
• If the user chooses to do so, they can set a max message size via AmqpTcpEndpoint.MaxMessageSize before creating a connection and it will be enforced.

[lukebakken]

Fixed by #1220