This is a BReference that allows only const access to the referenced
object. This was not easily possible with the existing BReference for
two reasons:
* BReference<const Type> would not work, as BReference needs to change
the reference count of the referenced object. Adding mutable and casting
where appropriate wouldwork but,
* It is now also possible to assign a BReference to a BConstReference
(to the same type, of course). The reverse is not allowed, making it
more difficult to "const cast" the referenced object (it's still
possible to "get" the object pointer and cast that).
BConstReference can be used to provide shared read-only access to an
object, for example this can be used to cache non-copiable or
expansive to create objects.
* The Network Kit now makes use of it for BUrlContext, so we need this
in the public headers.
* Problem caught by the new build bot by compiling the unit tests.
