lwm2m: keep track of observations per client

this has a number of advantages:
- allows to only create notifications for each client if there are no
  messages already waiting to be send, in practice prioritizing the
  memory for messages for answers, thus staying more "responsive".
- saves a fair bit of memory by eliminationg now redundant client_ctx
  pointer per observer.
- fixes a potential subtle bug: previously, an observer reset would've
  stopped the first observation found with a matching token, which
  might've belonged to a differen client.

Signed-off-by: Henning Fleddermann <henning.fleddermann@grandcentrix.net>