Add multitenancy

[michael-schnell]

Allow multitenancy for the event store commons.

• As this is mainly an implementation detail, it should have almost no effect on the API.
• Not all implementations will be capable to support this feature
• "Database multitenancy" is already available by simply using two instances of an event store.
• "Schema multitenancy" could be made available for JPA, Memory, ESJC and ESHTTP implementations
  • JPA might be difficult, because it depends on the implementation like Hibernate, EclipseLink, ...
  • ESJC and ESHTTP can support it by adding a tenant prefix to all streams
• An important topic would be security - One tenant should not be able to access the other tenant's data. Not sure if this works with JPA/Hibernate.