Simple modifications to the accounting clients

[fstagni]

With DIRAC v6r12 as target, we should modify the ReportGeneratorClient and the DataStoreClient so that they are capable to decide to which service to talk, depending on the accounting type.

This will enable the separation of WMSHistory from other accounting types as defined as highest priority point of https://github.com/DIRACGrid/DIRAC/wiki/Monitoring-service

[graciani]

Might be I'm wrong, but what we need is not a different Service but rather that the Service uses a different DB backend for a given Type. This implies a change on the server side and it is thus much more contained.

[acasajus]

It's been merged in #1934