Adding options to control scheduler multi-threading

Background/motivation: We are adding multi-threading to the scheduler. To that extent, I got feedback to add a manual, admin lever to control the number of threads that the scheduler will launch.

Pull request: https://github.com/PBSPro/pbspro/pull/1228

Forum discussion: http://community.pbspro.org/t/new-sched-attribute-to-control-scheduler-multi-threading/1737

Interface changes:

• New pbs_sched option '-t':
  
  • legal values: numeric >= 1.
  • Will be the total number of threads that the scheduler will create. If value is 1, then no new threads will be created, i.e - the main thread will run everything serially.
  • The scheduler will cap the number of threads at the core count value, even if the admin provides a number greater than that.
  • Can be set to different values for different scheduler instances if using multi-sched
  • Will be the "max" number of threads that the scheduler will create
• New pbs.conf & environment variable 'PBS_SCHED_THREADS':
  • legal values: numeric >= 1.
  • Will behave similarly to the pbs_sched -t option.
  • When set as an environment variable, PBS_SCHED_THREADS will take precedence over the pbs.conf value
  • The command line pbs_sched -t option will take precedence over PBS_SCHED_THREADS (as pbs.conf or environment variable)
  • Will apply to all multi-scheds using the same pbs.conf file.
• In the absence of pbs_sched -t & PBS_SCHED_THREADS, multi-threading will be disabled by default.