FlavourTagging persistency creates dangling references

The following discussion from !2688 (merged) should be addressed:

• @graven started a discussion:

  The newly cloned FlavourTag still refers to the original tagging particles -- this results (when this algorithm is used to copy FlavourTags for persistency) in dangling references, i.e. they point to containers not being persisted. If the intention is to not persist the referred particles, then one should at least call Tagger::clearTaggerParts() for all taggers in the FlavourTag pointed at by ft_clone.