Current implementation stores a copy of the logger as a pointer and
update its content, which is now unecessary since the introduction of
WithContext.
The new WithContext now takes the pointer and store it in the context if
none is stored already or if the last one stored is a different pointer.
This way it is still possible to update the context of a logger stored
in the context, but it is also possible to store a copy of the logger in
a sub-context.
Fix#80
