The native invocation in Read is locked per Stats object, since
VSC_Iter is not safe for concurrency with the same vsc and vsm
handles.

This is not necessary for the descriptions call, since it is only
invoked once per object at Attach time.

Use sync.Map for the global callback maps, and use 64 bits of
secure randomness for their keys (using the map length to generate
new keys was not safe for concurrency).

In rare cases, VSC_Iter may pass NULL as the pointer to a point,
so just skip it if that happens.