Enable built-in session cleanup in alpha and sandbox

App Engine provides a servlet which deletes up to 100 expired _ah_SESSION entities from DataStore. This CL adds a cron job to call the servlet every 15 minutes in both alpha and sandbox. Assuming all goes well, we will turn it on in production.

I originally learned about this servlet here:

http://www.radomirml.com/blog/2011/03/26/cleaning-up-expired-sessions-from-app-engine-datastore/

But it appears that we do not need a servlet definition, just a cron entry.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=137533532

java/google/registry/env/alpha/default/WEB-INF/cron.xml

@@ -166,5 +166,14 @@
     <target>backend</target>
   </cron>
 
+  <cron>
+    <url><![CDATA[/_ah/sessioncleanup?clear]]></url>
+    <description>
+      Delete up to 100 expired _ah_SESSION entities from Datastore.
+    </description>
+    <schedule>every 15 minutes</schedule>
+    <target>backend</target>
+  </cron>
+
 
 </cronentries>

java/google/registry/env/sandbox/default/WEB-INF/cron.xml

@@ -142,5 +142,14 @@
   </cron>
   -->
 
+  <cron>
+    <url><![CDATA[/_ah/sessioncleanup?clear]]></url>
+    <description>
+      Delete up to 100 expired _ah_SESSION entities from Datastore.
+    </description>
+    <schedule>every 15 minutes</schedule>
+    <target>backend</target>
+  </cron>
+
 
 </cronentries>