Access to key value pair in tag.Map needs to be thread-safe

[krnowak]

When calling Foreach() on the tag.Map, we gain access to every key and value in the map. We can also use Value() to get access to a value for a specific key. The keys and values returned should essentially be copies of whatever is contained in the map, and usually they are. There are some troublesome members though. Some of them have the pointer-like semantics, so it is possible to mutate them in the Foreach callback or just in the returned object from Value(), and thus mutate the contents of the map itself.

Currently that happens for the keys in the map that contain the BYTES Value. So the map implementation should make a copy of the bytes when giving the access to it.

I'm also wondering if the Type field in registry.Variable should also be copied, because it is an interface, so it also could have some pointer-like semantics when doing shallow copy during Foreach.

[jmacd]

Perhaps we should remove BYTES, that was suggested elsewhere.

[jmacd]

The Type variable could probably be eliminated. It's not supposed to contain any state.

[jmacd]

Oops. This is about allowing mutable value data into the SDK.

[jmacd]

I'd prefer to remove BYTES than to force a copy.