Revisit the way all simple types are passed around.

[fpacifici]

The simple types: Topic, Partition, Position are all cloned when being moved around the different layers of the library.
This is mostly not useful. Topics should have a static lifetime and passed by reference, Partition and Position should be owned by a consumer between one assignment and another and really may need to be cloned only when returning a message.

This means revisiting all the places where we pass and receive those structs and consider passing only references around instead of copying everything.