[core] Unique names for threads of Smack Cached Executor service

[guusdk]

Having unique names for threads of Smack's own executor service can help with debugging issues.

[Flowdalic]

I am a bit hesitant to add an atomic counter to thread creating. But maybe I am missing the motivation.

Looking at thread dumps, one sees

"Smack Cached Executor" #19
"Smack Cached Executor" #22 
"Smack Cached Executor" #26 

so yes, the thread names are not unique, but every thread has a unique ID. I never felt the need to make the thread names unique in such cases, so I am wondering about the motivation for doing so.

[guusdk]

Some debuggers do not show the thread ID, just the name. I forgot which one, but I ran into that, causing me to create this PR.

I think there's precedent in Smack for numbering threads in this way (on mobile now, hard to check). I checked - it doesn't.

What is your concern with atomics? Aren't they benign? Alternatively, we could concatenate the thread name with getId() I guess. Or we just drop this PR completely - I'm far from deeply invested in this one.

[Flowdalic]

Thanks for elaborating on it. I'd like to keep it as is. At least for now.