Handle PluginAttrInstances using ManagedStatic

We need to make sure that PluginAttrInstances is deleted before shared libraries
are unloaded, because otherwise when deleting its contents we'll try to access
a virtual destructor which no longer exists.

As shared libraries are managed using ManagedStatic we can do this by also using
ManagedStatic for PluginAttrInstances as ManagedStatics are deleted in reverse
order of construction and we know that PluginAttrInstances will only be
accessed, and thus constructed, after shared libraries have been loaded.