The premise of these changes is the fact that we cannot have
a cycle of #includes.
Because the declaration of `sender` is going to be moved to
a separate header file in a future commit, and because that
header file is going to be included in the file where
`end_point_hints_manager` is declared, we will need to rely
on `end_point_hints_manager` being an incomplete type there.
A consequence of that is that we cannot access any of
`end_point_hints_manager`'s methods.
This commit prepares the ground for it by moving
the definition of the function to the source file where
`end_point_hints_manager` will be a complete type.
1a7262ed6e