[Persistence] Reuse instances in cache

Reuse/update a single object instance in the persistence
cache, such that updates can be trivially reflected
everywhere. WTD-791.

platform/persistence/cache/src/CachingPersistenceDecorator.js

@@ -21,12 +21,40 @@ define(
             var spaces = CACHE_SPACES || [], // List of spaces to cache
                 cache = {}; // Where objects will be stored
 
+            // Update the cached instance of an object to a new value
+            function replaceValue(valueHolder, newValue) {
+                var v = valueHolder.value;
+
+                // If it's a JS object, we want to replace contents, so that
+                // everybody gets the same instance.
+                if (typeof v === 'object' && v !== null) {
+                    // Only update contents if these are different instances
+                    if (v !== newValue) {
+                        // Clear prior contents
+                        Object.keys(v).forEach(function (k) {
+                            delete v[k];
+                        });
+                        // Shallow-copy contents
+                        Object.keys(newValue).forEach(function (k) {
+                            v[k] = newValue[k];
+                        });
+                    }
+                } else {
+                    // Otherwise, just store the new value
+                    valueHolder.value = newValue;
+                }
+            }
+
             // Place value in the cache for space, if there is one.
             function addToCache(space, key, value) {
                 if (cache[space]) {
+                    if (cache[space][key]) {
+                        replaceValue(cache[space][key], value);
+                    } else {
                         cache[space][key] = { value: value };
                     }
                 }
+            }
 
             // Create a function for putting value into a cache;
             // useful for then-chaining.