PubSub: Use integer type for timestamps (SQL)

Store PubSub item creation/modification timestamps as integers instead
of "$megasec:$sec:$microsec" strings.  This can improve the performance
of certain SQL queries significantly.

Thanks to Ammonit Measurement GmbH for sponsoring this work.

sql/mssql.sql

@@ -210,8 +210,8 @@ CREATE TABLE [dbo].[pubsub_item] (
         [nodeid] [bigint] NULL,
         [itemid] [varchar] (255) NOT NULL,
         [publisher] [varchar] (250) NOT NULL,
-        [creation] [varchar] (32) NOT NULL,
-        [modification] [varchar] (32) NOT NULL,
+        [creation] [bigint] NOT NULL,
+        [modification] [bigint] NOT NULL,
         [payload] [text] NOT NULL DEFAULT ''
 ) TEXTIMAGE_ON [PRIMARY];