[WIP] External clients not receiving service-generated messages

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Original issue description:

In PyBlox, I have the following code (in a sprite):

device = '...'   # change these
password = '...' # change these

@onstart()
def my_onstart(self):
    nb.phone_iot.set_credentials(self.device, self.password)
    nb.phone_iot.listen_to_sensors(self.device, { 'accelerometer': 100 })

@nb.on_message('accelerometer')
def my_onmessage(self, x, y, z):
    self.say(x)

Nothing gets displayed, but the cloud server shows posts to /network/messages/ and NetsBlox/PyBlox-generated messages are correctly received in PyBlox (just not service-generated messages). It looks like what's happening is that the sendMessage feature on external client instances (which are used as the socket objects provided to services) are always explicitly constructing a browser target object, so lookup of the target fails and the message is dropped at cloud.

Fixes #270.

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