jobcomp/elasticsearch - save state on REQUEST_CONTROL.

jobcomp/elasticsearch saves/load the state to/from elasticsearch_state.  Since
the jobcomp API isn't designed with save/load state operations, the plugin
_save_state() isn't extern and not available from outside the plugin itself,
thus it is highly coupled to fini() function. This state doesn't follow the
same execution path as the rest of Slurm states, where in save_all_sate()
they are all independently scheduled. So we save it manually here on a RPC
of type REQUEST_CONTROL.

This enables that when the Primary ctld issues a REQUEST_CONTROL to the Backup
which is currently in controller mode, the Backup will save the state and when
the Primary assumes control again it can process the saved pending jobs.  The
other way around was already controlled, because when the Primary is running
in controller mode and the Backup issues a REQUEST_CONTROL, the Primary is
shutdown and when breaking the ctld main() function while(1) loop, there was
already a g_slurm_jobcomp_fini() call in place.

Bug 3908