Optionally dump statistics for a list of objects as part of statistics
logging.
If we fail to create the statistics cursor, we may still have allocated
a statistics structure for it.
Now there are multiple sets of objects, sort them so objects group
together.
Replace statistics_log.objects with statistics_log.sources, no other change.
Merge pull request #489 from wiredtiger/stat-objects

Add automatic logging of statistics for data sources.