Moved settings cache to an explicit "in memory" adapter

refs https://github.com/TryGhost/Toolbox/issues/364

- Before introducing an image size caching mechanism we need to make existing caches explicit - makes sure caches are operating in multiple different instances.
- Explicit syntax also allows to pass in different configurations for each cache instance (e.g. use different Redis connections for caches with different purposes)

ghost/core/core/server/adapters/cache/SettingsCacheSyncInMemory.js

@@ -0,0 +1,7 @@
+const Memory = require('./Memory');
+
+class SettingsCacheSyncInMemory extends Memory {
+
+}
+
+module.exports = SettingsCacheSyncInMemory;

ghost/core/core/server/services/settings/settings-service.js

@@ -67,7 +67,7 @@ module.exports = {
      * Initialize the cache, used in boot and in testing
      */
     async init() {
-        const cacheStore = adapterManager.getAdapter('cache');
+        const cacheStore = adapterManager.getAdapter('cache:settings');
         const settingsCollection = await models.Settings.populateDefaults();
         SettingsCache.init(events, settingsCollection, this.getCalculatedFields(), cacheStore);
     },

ghost/core/core/shared/config/defaults.json

@@ -25,7 +25,9 @@
             "active": "Default"
         },
         "cache": {
-            "active": "Memory"
+            "active": "Memory",
+            "settings": "SettingsCacheSyncInMemory",
+            "SettingsCacheSyncInMemory": {}
         }
     },
     "storage": {