Add checked move

[redboltz]

No description provided.

[redboltz]

I'm not sure the name of the function force_move() is appropriate?
Other choice is checked_move(). Or something else...

[jonesmz]

I'm not sure I understand the goal of this change.

Just to ensure that std::move() is only called on things that will actually be moved?

Is there a bug?

Or is this to just make the code easier to understand?

[redboltz]

See https://stackoverflow.com/questions/57105812/how-to-detect-unexpected-copy-in-move-expected-case/57106531#57106531

std::move(v) invokes copy constructor if v is not movable but copyable.
But in mqtt_cpp, I don't expect such behavior. When I call std::move(), I always expect move operation. So I need to avoid fallback copy operation. The PR does it.

The PR contains two commits. The first one is introduced wrapped move. Then I got some unexpected copy fallbacks. The second commit fixed them.

[jonesmz]

Alright, that's reasonable

[redboltz]

https://github.com/redboltz/mqtt_cpp/pull/378/files#diff-45ca9626e6c7c611be2ceb2ff63031a5R470

I replaced move with copy at the point above.

See
https://stackoverflow.com/questions/57335172/is-there-any-way-to-move-an-element-from-ordered-index-ed-multi-index

In modify() function, if the element in the container is moved, then access moved from object to re-arrange elements.