Multi index support

[alexander-schranz]

To implement multiple indexes without any sideeffects we should create per Index an own doctrine entity manager and add its custom MetaDataLoader which add a table prefix.

http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/cookbook/sql-table-prefixes.html

The services (repositories, transaction manager, search executer, index) need to be generated in PHP per index.