Cache redesign part 1, and add support for alternative links

src/app/core/cache/server-sync-buffer.effects.spec.ts

@@ -51,7 +51,7 @@ describe('ServerSyncBufferEffects', () => {
               });
               return observableOf(object);
             },
-            getBySelfLink: (link) => {
+            getByHref: (link) => {
               const object = Object.assign(new DSpaceObject(), {
                 _links: {
                   self: { href: link }