Observer's subscription ID shouldn't be required

[davidbrochart]

In the following code we register a callback for an observer and don't keep a reference to the subscription ID:

import y_py as Y

d = Y.YDoc()

def callback(e):
    print(e)

x = d.get_text("test")

def register_cb(x, callback):
    i = x.observe(callback)
    # if i is not returned, the callback will never be called
    # return i  # this works fine (callback called)

i = register_cb(x, callback)

with d.begin_transaction() as txn:
    x.insert(txn, 0, "abcd")

This can happen when we don't want to unobserve. But the issue is that in this case the callback is never called.