Set rpc status while deleting
We're getting the following error, showing that rpc.active decreases
below zero.  This change is just a blind guess as to the cause.
File "C:\projects\distributed\distributed\utils_test.py", line 326, in     check_active_rpc
assert rpc.active == rpc_active
AssertionError: assert -1 == 0
@@ -563,6 +563,7 @@ class rpc(object):
def __del__(self):
if self.status != 'closed':
rpc.active -= 1
+ self.status = 'closed'
n_open = sum(not stream.closed() for stream in self.streams)
if n_open:
logger.warn("rpc object %s deleted with %d open streams",
