zebra: label manager refactor

in order to both streamline the code and allow users to
define their own specialized versions of the LM api handlers,
define hooks for the 4 main primitives offered by the label
manager (i.e. connect, disconnect, get_chunk and release_chunk),
and have the existing code be run in response to a hook_call.
Additionally, have the responses to the requesting daemon be
callable from an external API.

Note that the proxy version of the label manager was a source of
issues and hardly used in practice. With the new hooks, users with
more complex requirements can simply plug in their own code to
handle label distribution remotely, so there is no longer a reason
to maintain this code.

Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>

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