linuxbridge-agent: process port updates in the main loop

This patch changes the way the linuxbridge agent processes port update
notifications. It does the same for the lb-agent as was done for the
ovs-agent in I219c6bdf63b0b5e945b655677f9e28fa591f03cd.
Now the RPC call just adds the updated tap device name to a set of
updated devices, and the actual processing is done in the main RPC loop.
This should solve the problems were port_update RPCs were competing
with the main loop/greenthread and with each other to do the same task
- like adding a tap or interface to a bridge - which lead to races and
was inefficient. Now repeated port_update notifications received within
a single main loop iteration will be coalesced and processed only once.

Closes-Bug: 1256950
Change-Id: I7fd48542f12b39ffc1346d1a6c9a387ecda6d812
Removed create_rpc_dispatcher methods

Now that we don't have a special dispatcher class and we pass a list of
endpoints to corresponding functions instead, those methods are
unneeded.

blueprint oslo-messaging

Change-Id: If2b187fd8e553594212264f34b51b5b99c4630b2
Removed 'rpc' and 'notifier' incubator modules

Port to oslo.messaging is done, so remove now unused pieces from the
tree.

Also removed obsolete bin/quantum-rpc-zmq-receiver. It comes from old
days of oslo-rpc, it was reimplemented later as
openstack/common/rpc/zmq_receiver.py in oslo-incubator, and we've
removed the whole tree for old RPC layer anyway.

blueprint oslo-messaging

Change-Id: If7155c59c8ef58d06164938998180f3367b9bb16
Merge "Removed create_rpc_dispatcher methods"
Merge "Removed 'rpc' and 'notifier' incubator modules"
Merge "linuxbridge-agent: process port updates in the main loop"