add a threading hub module

this provides gevent-like api using eventlet.
sweep the tree to change from gevent to ryu.lib.hub

mostly mechanical changes.
also, change the requirement from gevent to eventlet.
ovs.poller: workaround a bug in eventlet
hub: missing else
context proxy
controller: fix send queue draining again

and this time add comments to explain the intention.
limit the size of ryu app's event queue

otherwise a queue grows too long and consumes much memory on load.
the size used in this commit (128) is arbitrary.
vsctl test code from yamahata
ovs_vsctl.py: rebase and tweaks
ovs_vsctl.py: print timing info
add a ryuapp to provide python interactive console for debugging
hub: fix event.wait timeout
ovs_vsctl: pep8
rename context_proxy to object_proxy
app_manager:
follow object_proxy rename
proxy only if _CAN_PROXY
object_proxy: bug fixes
_CAN_PROXY for network