Supported added for new RPC EPILOG_COMPLETE. Slurmd now responds immediately

to job_kill request and slurmctld leaves node and job in COMPLETING state
until the slurmd issues an EPILOG_COMPLETE RPC on each node. This permits
better support for non-killable processes and/or long-running epilog scripts.
Several minor changes in node registration handling and slurmctld agent
logic to better address a flood of incomming RPC (typically when system
restarts).
(gnats:268)