Add isConnected method

Author: AndrewJo

src/driver.ts

@@ -7,6 +7,11 @@
  * Abstract base class required for implementing a cache driver.
  */
 export default abstract class Driver {
+  /**
+   * Returns whether the driver is connected to the underlying backend.
+   */
+  public abstract async isConnected(): Promise<boolean>;
+
   /**
    * Lists all keys.
    */

src/drivers/memory.ts

@@ -14,6 +14,10 @@ export default class Memory extends Driver {
     this.cache = new NodeCache();
   }
 
+  public async isConnected(): Promise<boolean> {
+    return true;
+  }
+
   public async keys(): Promise<string[]> {
     return this.cache.keys();
   }