Job coalesce detection

This branch adds handling for coalesced jobs. The job_guid construction function `find_job_guid()` now handles multiple request ids and request times and generates a list of coalesced job guids. This information is stored in the job json structure and stored in the database. The column data in `job.job_coalesced_to_guid` is returned to the UI to allow hiding coalesced jobs in the main view, but the list could be displayed in the detail panel if that seems useful.