Don't look at all the threads before each GC.

Don't look at all the threads before each GC.
We were looking at all the threads for 2 reasons:
 1. to catch transactions that might be looping as a
    result of seeing an inconsistent view of memory.
 2. to catch threads with blocked exceptions that are
    themselves blocked.
For (1) we now check for this case whenever a thread yields, and for
(2) we catch these threads in the GC itself and send the exceptions
after GC (see performPendingThrowTos).