Define new RPC REQUEST_KILL_TIMELIMIT, use same data format as REQUEST_REVOKE_JOB_CREDENTIAL.

slurmctld sends REQUEST_KILL_TIMELIMIT when job reaches its time limit,
change job state to TIMEOUT | COMPLETING and node state to COMPLETING.
slurmd upon receipt of REQUEST_KILL_TIMELIMIT sends job SIG_XCPU and SIGKILL
then runs epilog.