Correctly initialize comms and connect streams

[philippjfr]

This PR ensures that only the top-level plot instance creates a Comm instance and then attaches the plot.refresh method as a _hidden_subscriber to the stream. A simple test I used:

posx = hv.streams.PositionX()
dmap = hv.DynamicMap(lambda x: hv.Image(np.random.rand(x+1,10)), kdims=[], streams=[posx])

dmap + hv.Image(np.random.rand(10,10))

for i in range(10):
    posx.update(x=i)

The plot updates in response to the position updates.

[jlstevens]

Looks good. Merging.