7034139: G1: assert(Thread::current()->is_ConcurrentGC_thread()) failed: only a conc GC thread can call this. Summary: We were calling STS join and leave during a STW pause and we are not suppoesed to. I now only call those during concurrent phase. I also added stress code in the non-product builds to force an overflows (the condition that ws uncovering the bug) to make sure it does not happen again. Reviewed-by: johnc, brutisso