hub: make eventlet version of joinall similar to gevent one

to align gevent's joinall which does raise_error=False by default,
ignore exceptions for eventlet version.
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.