Use Map to store attributes

[reklatsmasters]

Any attribute type MAY appear more than once in a STUN message.
Unless specified otherwise, the order of appearance is significant:
only the first occurrence needs to be processed by a receiver, and
any duplicates MAY be ignored by a receiver.

https://tools.ietf.org/html/rfc5389#section-15

• Use Map to store only the first attribute?
• Add [[readonly]] flag for any arrived messages?
• Do not allow to add more than once any attribute?