Revert "Protocol.receive: bump up threads from 10 to 30"

Seemed to make things worse, not better. Guess I need to implement continuation passing across task handler requests. #652

This reverts commit 66da2efc7a.

protocol.py

@@ -38,7 +38,7 @@ SUPPORTED_TYPES = (
     'video',
 )
 
-DELIVER_THREADS = 30
+DELIVER_THREADS = 10
 
 # activity ids that we've already handled and can now ignore.
 # used in Protocol.receive