Make message type final in InfoMessage

The setter was only used during initialization. This patch makes the
field final and sets the value through the constructor. This makes it
simpler to enumerate all possible values in a parser for billing files.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6074/