Can no longer close server via RPC with loopless design

Before the new loopless design, a client could close a server through an RPC call and receive a response to the call. After switching to the loopless design in 0.5, the server exits, but the client doesn't receive a response.

With the old design, I could ensure that the server responded before closing by scheduling the `server.close()` call with `loop.call_soon`. This is no longer sufficient unfortunately.
## Server

```
import asyncio
import aiozmq.rpc

class ServerHandler(aiozmq.rpc.AttrHandler):
@aiozmq.rpc.method
def exit(self):
loop = asyncio.get_event_loop()
loop.call_soon(self.server.close)
print('Service shutting down...')

@asyncio.coroutine
def go():
handler = ServerHandler()
server = yield from aiozmq.rpc.serve_rpc(
handler, bind='tcp://127.0.0.1:5555')
handler.server = server
yield from server.wait_closed()

asyncio.get_event_loop().run_until_complete(go())
```
## Client

```
import asyncio
import zmq
import aiozmq.rpc

@asyncio.coroutine
def go():
client = yield from aiozmq.rpc.connect_rpc(
connect='tcp://127.0.0.1:5555')
# Tell zmq not to wait until buffered data is sent after socket is closed
client.transport.setsockopt(zmq.LINGER, 0)

ret = yield from client.call.exit()
client.close()

asyncio.get_event_loop().run_until_complete(go())
```