'idempotentFullSync' method to synchronize collections

[andrus]

recent "createOrUpdate" functionality in LinkRestService doesn't go far enough. It doesn't allow to build a true PUT request on a collection (e.g. "update the state of 1..N"). It is half way there, but deleting items is not supported. Need to add another Java operation (preliminary name: "idempotentFullSync") that would treat incoming data as a full collection representation and hence would delete server items not in the collection.

There's synergy with #29 - once we switch to multi-object locators, it will be easy to implement delete.