Call_provers:

- drop closures "pre_prover_call" and "post_prover_call". They were
  intended to be used for synchronous interaction with provers from
  multiple threads. This is now responsibility of the proof server:
  (a) any Call_prover.call_on_[file|buffer] submits the proof task
      immediately to the server;
  (b) all proof results are handled in the working Why3 thread.

- Call_provers.query_call returns a tri-state "prover_update" type
  which can be one of: "ProverStarted" (returned after the proof
  server informs Why3 that a prover was started), "ProverFinished"
  (returned after the proof server returns the prover result), and
  "NoUpdates" (returned when the proof server has not sent any new
  updates concerning the proof task in question).

    IMPORTANT: query_call does not maintain the state of a given
  prover call. In a normal use case, "ProverFinished" is returned
  _exactly_ once, and "ProverStarted" _at_most_ once (never for
  an editor call or when rapidly overwritten by "ProverFinished").

    TODO: extend the proof server protocol and implementation to
  send "ProverStarted" events back to Why3.