improvement: allow setting a Message `Key` as `AddValue` parameter about libkafka-asio

[danieljoos]

I fully agree. This one needs to go away. Probably better to just have AddMessage!?

[DEvil0000]

I don't like the AddMessage method signature.

1. you have to create a Message, call some methods to set all attributes.
2. depending on the use case you have to create a MessageSet then.
3. then you pass the object to the AddMessage method which makes a deep copy of it.
4. then you have to clean up the MessageSet and the Message
5. after that the lib will clean up the same information again.
   this sounds like overhead. calling much more methods then needed. and doing some memory allocations we don't need (2* MessageSet and the Messages).
   I think it would be better to improve the AddValue method and maybe rename it to AddMessage. We know all components of a Message and they are just a few:

• topic (may be given by config on produce/producer level)
• partition (may be given by config on produce/producer level)
• key (optional)
• value
  the rest is calculated or can be set by config