controller: fix send_q draining

when stopping the sender thread, ensure that no one is going to block
on send_q.  otherwise a ryu app who wants to do send_msg on the datapath
can block on the queue forever if the queue is full.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>