Add No Touching

This adds `#no_touching` on all ActiveRecord models, allowing to ignore touching for the duration of a block.

## Example:

    ActiveRecord::Base.no_touching do
      Post.first.touch #=> Does nothing
    end

    Comment.no_touching do
      Comment.first.touch #=> Does nothing
      Post.first.touch #=> Updates, but won't update related comments if there are any
    end

cc @dhh