smtk::attribute::ReferenceItem and smtk::resource::Surrogate are
designed to use weak pointers to resources as caching mechanisms to
prevent repeated lookups in smtk::resource::Manager and to facilitate
their use in a manager-less design (e.g. for scripting). There is
currently a bug in SMTK/ModelBuilder where there are too many shared
pointers to a loaded resource; when a resource is removed from the
system, it persists in memory due to these untracked shared
pointers. This results in the caching weak pointers never being
invalid, so cache entries do not become stale and refresh via the resource
manager. Until we track down all of the stray shared pointers to
resources, this MR forces the caches in ReferenceItem and Link to
refresh whenever they are accessed.