Refactor analytics.gather

With the change to use pk-based interval slicing for the job events
table, we need analytics.gather to be the code that manages all of the
"expensive" collector slicing.  While we are at it, let's ship each
chunked tarball file as we produce it.

awx/settings/defaults.py

@@ -620,6 +620,8 @@ INSIGHTS_TRACKING_STATE = False
 
 # Last gather date for Analytics
 AUTOMATION_ANALYTICS_LAST_GATHER = None
+# Last gathered entries for expensive Analytics
+AUTOMATION_ANALYTICS_LAST_ENTRIES = None
 
 # Default list of modules allowed for ad hoc commands.
 # Note: This setting may be overridden by database settings.