Use a single sender
Use a single sender

Instead of register many different senders (one for each needed
proxy notification type) just use a single sender for all the
different types to save space and time when the senders are
pickled across to the worker(s).

Change-Id: I1e1ab6c708c855e5868061bc338d399a38ad954e

pombredanne/taskflow-1,openstack/taskflow,openstack/taskflow,pombredanne/taskflow-1,junneyang/taskflow,jimbobhickville/taskflow,junneyang/taskflow,jimbobhickville/taskflow