No longer starting a goroutine -- users decide if they want one.

Native reads are locked per Log object, since they are not safe for
concurrency with the same native handles.

The global callback map is keyed with an atomically incremented uint64.

The VSM handle is exported as a field in struct VSM, for internal
use only.

Names and descriptions are stored per object, no need to sync on the
global map after Attach.

Attach() takes one argument for the instance, default for "", otherwise
the named instance.

Remove AttachInstance() and the progress argument.

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.