Add an index to cover the expire SQL query

appinfo/database.xml

@@ -103,6 +103,14 @@
 				<length>4</length>
 			</field>
 
+			<index>
+				<name>activity_time</name>
+				<field>
+					<name>timestamp</name>
+					<sorting>descending</sorting>
+				</field>
+			</index>
+
 			<index>
 				<name>activity_user_time</name>
 				<field>

appinfo/info.xml

@@ -15,7 +15,7 @@
 	<licence>AGPL</licence>
 	<author>Frank Karlitschek, Joas Schilling</author>
 	<shipped>true</shipped>
-	<version>2.2.0</version>
+	<version>2.2.1</version>
 	<default_enable/>
 	<types>
 		<filesystem/>