Currently, a view update backlog may reach an invalid state, when
its max is 0 and its relative_size() is NaN as a result. This can
be achieved either by constructing the backlog with a 0 max or by
modifying the max of an existing backlog. In particular, this
happens when creating the backlog using the default constructor.
In this patch the the default constructor is deleted and a check
is added to make sure that the max is different than 0 is added
to its constructor - if the check fails, we construct an empty
backlog instead, to handle the possibility of getting an invalid
backlog sent from a node with a version that's missing this check.
Additionally, we make the backlogs members private, exposing them
only through const getters.
cde14a5788