eliminate flux_reactor_active_incref()

[garlick]

Problem: flux_reactor_active_incref() and flux_reactor_active_decref() have no users and have a questionable design:

• From 5bd6620: they only work with certain watchers

  The use of flux_reactor_active_incref/decref() only works with
  simple watchers that only implement a single watch. More complex
  watchers, which may have multiple watchers internal to it, cannot
  use this mechanism.

• From libflux/reactor: add flux_reactor_active_incref(), _decref() #2387: the interface is lame

  My only worry is that incref/stop phase of this approach will be pretty easily missed.

• From replace libev with libuv #6492: they cannot be provided if we swap out libev for libuv

I would vote that we remove these from the public API now and if the need arises for them again, we try to implement an interface like the per-watcher ("handle" in libuv speak) uv_ref().

[garlick]

I missed that this is used in flux-start(1) and in the python bindings in handle.py::reactor_run().

The flux-start usage is easily worked around. The python one, added in c1afc01, might require a little more thought, or possibly argue for the uv_ref() style interface above.