[Loggable] introduce a combined search index for ext_log_entries to increase performance

Performance of entity updates is VERY slow when ext_log_entries reaches a sitze of several 100.000 entries. The query for MAX(version) takes typically over 0.5 seconds. With this combined index the same query takes about 0.004 seconds. If you have several dozend entities being updated, this will result in a huge performance increase - with the cost of more space needed for your db table, of course.