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).