Improved index creation on dynamic_field_value table to better support searches.

Squashed commit of the following:

commit 5000aebd23fe5c43145cbcfaa6f1399d40869be9
Author: Martin Gruner <martin.gruner@otrs.com>
Date:   Thu Jan 23 17:50:40 2014 +0100

    Fixed typo.

commit 74f8c9175b5cb8cd17c152a5680621714b169381
Author: Martin Gruner <martin.gruner@otrs.com>
Date:   Thu Jan 23 17:42:40 2014 +0100

    Improved index creation on dynamic_field_value table to better support searches.