CI Hanging on GC Tests Fix

[1stub]

I believe this fixes the issue with CI hanging when running GC tests. After some thought, it appears using std::jthread for our decs thread is problematic as join is called on these at the destruction of a BSQMemoryThreadLocalInfo object, not necessarily when we signal from main that our program is done.

So, we now use std::thread and (in DecsProcessor::signalFinished()) once the worker thread has ack'd completion we call an explicit join.

I tested this locally using act and it appears to be fixed, so hopefully this is reflected in github CI.

[1stub]

After being confused for a while I realized there were some flaws in my state management logic between threads causing decrement thread state to not be communicated well. This lead to infinite waiting.

My new changes shifted towards a more organized decs thread state management and less complex conditionals. Instead of using multiple flaky booleans we use a singular state enum (which I should have done from the start).

I tested this by making a pr within my fork to trigger ci and we are passing there, so things should be good now. Multi-threadded programming is tricky!