Add thread traces to USR1 handler

When receiving a USR1 signal, it did usefully provide
'the' current stack, useful for diagnosing really hard
hangs.  However, it's frequently informative to see all
the thread stack traces, so add that data to the diagnostic
feature.

confluent_server/confluent/main.py

@@ -50,6 +50,8 @@ try:
 except ImportError:
     havefcntl = False
 #import multiprocessing
+import gc
+from greenlet import greenlet
 import sys
 import os
 import signal
@@ -129,6 +131,13 @@ def dumptrace(signalname, frame):
     ht = open('/var/log/confluent/hangtraces', 'a')
     ht.write('Dumping active trace on ' + time.strftime('%X %x\n'))
     ht.write(''.join(traceback.format_stack(frame)))
+    for o in gc.get_objects():
+        if not isinstance(o, greenlet):
+            continue
+        if not o:
+            continue
+        ht.write('Thread trace:\n')
+        ht.write(''.join(traceback.format_stack(o.gr_frame)))
     ht.close()
 
 def doexit():