burst_buffer/cray work

Remove redundant persist_rem field from job record
Fix logic for buffer state related to failure handling
Move pre_run logic into a separate thread for improved system performance
Disable changes to job burst buffer field
burst_buffer contents are validated at job submit time and
  data is possibly being staged at later times. It can not
   be changed.
Refactor bb_p_job_cancel() logic
Fix some job staging problems
Eliminate logging of unchanged pool information
Document that job arrays don't support burst buffers
Refactor job buffer allocation to eliminate race condition with
  stage out. Needs more work after prolog_running functionality
  is in place (merged from v14.11).
Improve cold-start state clean up