Add "show threadpool calls" command to show count of all labelled smartthreadpool calls
Label all threadpool calls being made in core OpenSimulator.  This is to add problem diagnosis.

"show threadpool calls" now also returns named (labelled), anonymous (unlabelled) and total call stats.
Add naive implementation of controlled incoming HG attachments to manage load.

Instead of processing all incoming attachment scene object concurrently, process them consecutively to eliminate potential overload from this source.
This is a naive implementation because it does not currently account for slow foreign asset services.
Although it may take longer, this approach may also improve attachment visibility for HG avatars
since the scene object is now always added to the scene after receiving assets from the foreign service and not before.
Add "show threadpool calls active" console debug command.

This shows named threadpool calls (excluding timer and network calls) that are currently queued or running.
Also shows total of labelled and any anonymous calls.