Delay broadcasting Channel Updates until connected to peers

Delay broadcasting Channel Updates until connected to peers

    - We might generate channel updates to be broadcast when
      we are not connected to any peers to broadcast them to.
    - This PR ensures to cache them and broadcast them only when
      we are connected to some peers.

Other Changes:
    1. Introduce a test.
    2. Update the relevant current tests affected by this change.
    3. Fix a typo.
    4. Introduce two functions in functional_utils that optionally
       connect and disconnect a dummy node during broadcast testing.