examples/slurm: wait for the DVM to resize, and make cancel usable

[Petter-Programs]

Problem

examples/slurm/elastic-coordination.c treated the PMIx allocation callback as
the final answer to a size change. A Slurm extend now answers in two phases, so
the example reported success while the nodes it had just been granted could not
yet carry work. It also had no way to withdraw a pending request, and kept an
allocation request's wait object on the caller's stack.

Changes

examples/slurm: keep an allocation request's wait object alive

A timeout does not withdraw the request: PMIx still runs the callback whenever
the scheduler answers, by which time the submitting call may have returned and
taken its stack frame with it. The wait object is reference counted now.
Pre-existing bug, so it lands separately.

examples/slurm: wait for the DVM to resize, and make cancel usable

• Registers for PMIX_DVM_IS_READY / PMIX_ERR_DVM_MOD, correlating by
  PMIX_ALLOC_REQ_ID and claiming the slot before submitting — nothing orders
  the event after the phase-one callback.
• Watches each request on its own thread so the prompt returns at once. This is
  what makes cancel reachable: ras/slurm drops a request's cancellable
  record when the scheduler grants, and phase one does not answer until then,
  so waiting inline held the prompt for exactly as long as the request could
  still be withdrawn.
• Adds cancel [REQUEST_NAME] and pending, and extend count N as REQUEST_NAME
  so the name is short enough to type inside that window.
  Generated names count requests in the session (extend-1, shrink-2).
• Only an extend is cancellable, so only an extend takes a name; a cancel
  naming anything else is refused locally rather than sent to return
  PMIX_ERR_NOT_FOUND, which is indistinguishable from a typo.
• Verifies an extend against the allocation its completion event names rather
  than the DVM total, which is a snapshot any concurrent request invalidates.
• Refuses to start outside a Slurm job, or when PRRTE does not already know
  that job's allocation.
• Serializes output behind a lock, and silences the verify polls.