dr_app_cleanup should not kill the other threads

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on August 19, 2013 17:43:26_

Currently, dr_app_cleanup will kill all the other threads (in debug build or with a client wanting synchall), and generally assume the process is going down. We should support the app continuing on without DR. Furthermore, the current implementation is unsafe, as killing the other threads without the current thread immediately exiting (like it does under non-start/stop DR) can cause problems where the remaining thread waits on a lock owned by a killed thread.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1248_