controller: fix send queue draining again

and this time add comments to explain the intention.

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

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