add progress log to JHS during startup

lWhen the JHS starts up, it initializes the internal caches and storage via the HistoryFileManager. If we have a large number of existing finished jobs then we could spent minutes in this startup phase without logging progress:
2016-03-14 10:56:01,444 INFO org.apache.hadoop.mapreduce.v2.jobhistory.JobHistoryUtils: Default file system [hdfs://hadoopcdh.itnas01.ieee.org:8020]
2016-03-14 10:56:11,455 INFO org.apache.hadoop.mapreduce.v2.hs.HistoryFileManager: Initializing Existing Jobs...
2016-03-14 12:01:36,926 INFO org.apache.hadoop.mapreduce.v2.hs.CachedHistoryStorage: CachedHistoryStorage Init
This makes it really difficult to assess if things are working correctly (it looks hung). We can add logs to notify users of progress.