Added basic memory cache storage implementation

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

- This is groundwork to substitute in memory caches we use across the codebase. The first candidate would be settings cache. The interface of the memory cache was kept to the minimum. The "keys" method is a somewhat acceptable alternative to the "getAll" method used in the codebase right now.
- The next iteration over this would be adding async methods are alternative key/value storage methanisms like Redis.

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

@@ -0,0 +1,12 @@
+class Cache {
+    constructor() {
+        Object.defineProperty(this, 'requiredFns', {
+            // NOTE: "keys" method is only here to provide smooth migration from deprecated "getAll" method
+            //       once use of "getAll" is eradicated, can also remove the "keys" method form the interface
+            value: ['get', 'set', 'reset', 'keys'],
+            writable: false
+        });
+    }
+}
+
+module.exports = Cache;

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

@@ -0,0 +1,36 @@
+const Base = require('./Base');
+
+class MemoryCache extends Base {
+    constructor() {
+        super();
+
+        this._data = {};
+    }
+
+    get(key) {
+        return this._data[key];
+    }
+
+    /**
+     *
+     * @param {String} key
+     * @param {*} value
+     */
+    set(key, value) {
+        this._data[key] = value;
+    }
+
+    reset() {
+        this._data = {};
+    }
+
+    /**
+     * Helper method to assist "getAll" type of operations
+     * @returns {Array<String>} all keys present in the cache
+     */
+    keys() {
+        return Object.keys(this._data);
+    }
+}
+
+module.exports = MemoryCache;

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

@@ -0,0 +1,17 @@
+const adapterManager = require('../../services/adapter-manager');
+
+/**
+ * @param {'settings'|'theme'|'urls'} [feature] - name for the "feature" to enable through adapter, e.g.: settings cache
+ * @returns {Object} cache adapter instance
+ */
+function getCache(feature) {
+    let adapterName = 'cache';
+
+    if (feature) {
+        adapterName += `:${feature}`;
+    }
+
+    return adapterManager.getAdapter(adapterName);
+}
+
+module.exports.getCache = getCache;

ghost/core/test/unit/server/adapters/cache/Memory.test.js

@@ -0,0 +1,29 @@
+const assert = require('assert');
+
+const MemoryCache = require('../../../../../core/server/adapters/cache/Memory');
+
+describe('In Memory Cache Adapter', function () {
+    let memoryCache;
+
+    beforeEach(function () {
+        memoryCache = new MemoryCache();
+    });
+
+    it('stores a value through set method', function () {
+        memoryCache.set('a', 'Alabama');
+
+        assert.deepEqual(['a'], memoryCache.keys());
+        assert.equal('Alabama', memoryCache.get('a'));
+    });
+
+    it('flushes the storage', function () {
+        memoryCache.set('t', 'Texas');
+
+        assert.equal('Texas', memoryCache.get('t'));
+
+        memoryCache.reset();
+
+        assert.deepEqual([], memoryCache.keys());
+        assert.equal(undefined, memoryCache.get('t'));
+    });
+});