Improve Synchronizer handling

• The constructor is now private and can only be called by the SynchronizerManager

• Building of exchange info is cleaner: checking if the lists have been build (on-demand) is no more obtained by checking if the lists are empty, one now use a unique pointer that is filled at construction. This strategy is safer (for instance, a processor could be empty which would generate a code crash in the previous implementation).

enabled an automatic merge when the pipeline for 67a07a43 succeeds

merged

mentioned in commit 5d0d902f