initial tests on observe

checking in this state as it shows an interesting behavior:

* the client-side observation cancellation (which currently means
  silently removing the observation, resulting in an RST to be responded
  to the next update) results in the server to send an RST back. (afaik
  it shouldn't do that).
* even though the client-side observation cancelled itself, it still
  receives the RST, resulting in cancel() to be called again. (the
  `assert` is rightfully in line 1125: we don't expect the network not
  to send anything any more. we do expect the Protocol not to bother
  this observation again.
(minor debugging enhancement)
notes and minor fixes for observation handling
observations: exchange monitors need to be prepared that their observation is already cancelled
cancel observations on server shutdown
more observe tests, comment on lack of cancellation
Merge branch 'observetests'