hooks: add a prewlock and a prelock hooks This is useful for testing.

exchange: fix locking to actually be scoped The previous code was taking locks before entering with statements, so exception before the with statement would not release the lock (except for garbage collection). We need to move to a try except here because the logic is more complicated.

exchange: fix locking to actually be scoped The previous code was taking locks before entering with statements, so exception before the with statement would not release the lock (except for garbage collection).