asynch signal interactions with client clean calls

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on October 02, 2011 20:07:11_

delayable signal delivery time may be unbounded if arrives in middle of
client clean call since won't unlink underlying fragment unless signal
arrives directly inside it. not a problem for base DR, only with client
inserting clean calls.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=569_