ItemSync: use RID merge by default, allow optional switch to GID merge

GID is not guaranteed to be actually unique and is not in our control, which means
that we can easilly run to the multiple merge candidates problem, which breaks
ItemSync. This patch switching ItemSync to use RID, which (although not enforced) is
basically guaranteed to be unique per Collection, thus the multiple merge candidates
error should not happen anymore.

Some resources however mandate GID-based merging (when RID is not stable), so we have
a new API that allows resources to switch the merging mode of ItemSync optionally.

(cherry picked from commit 44296191)