Make DataSvc thread safe

[tmadlener]

As already discussed in #94 eventually we want to move to Gaudi Functional. This requires that our DataHandle / DataSvc works in a thread safe way. #100 introduces Frame based I/O but probably not yet in a thread safe enough way to make it work inside Gaudi Functional.

[tmadlener]

@jmcarcell do you want to keep this open until this is done "properly", or can we close this?

[jmcarcell]

I don't mind either. DataHandles are not used with Gaudi::Functional so the title is a bit wrong on that but it's true that the DataSvc is not thread-safe. And there is a third topic which is moving to Gaudi::Functional which is somehow independent of the other ones (but not complete if the DataSvc is not thread-safe)

[jmcarcell]

This is now completed by having a new service that can be used with multithreading IOSvc.