Performance Optimization for Loading Jobs Initially

This pull request optimizes the loading of jobs. When there are more than 20,000 jobs in jenkins the previous file-filter based check happened in a single thread and was taking in order of minutes to do the check.  This moves the config file check in file filter for listing of jobs (presence of a config file under a directory in jobs) as part of loading job so that it is done in multi-thread fashion than in a single thread. It also does only the config file check  which is sufficient to check job being a directory also (as the job being directory check is redundant and not required). This also reduces the number of file I/O ops by half.