propagate the enableMT flag from DQMStore to MEtoEDMConverter and DQMRoo...

...tOutputModule and make the DQMRootSource a friend with DQMStore
If you added a public function
```
bool multiThreadingEnabled() const { return enableMultiThread_; }
```
Then you wouldn't have to make DQMRootSource a friend. That would minimize coupling and make maintenance easier.