Enhance DatabaseQueue automatic memory management

We have async methods, now: no need to use a global dispatch queue.

Author: groue

GRDB/Core/DatabaseQueue.swift

@@ -119,21 +119,21 @@ extension DatabaseQueue {
 
         let task: UIBackgroundTaskIdentifier = application.beginBackgroundTask(expirationHandler: nil)
         if task == .invalid {
-            // Perform releaseMemory() synchronously.
+            // Release memory synchronously
             releaseMemory()
         } else {
-            // Perform releaseMemory() asynchronously.
-            DispatchQueue.global().async {
-                self.releaseMemory()
+            // Release memory asynchronously
+            writer.async { db in
+                db.releaseMemory()
                 application.endBackgroundTask(task)
             }
         }
     }
 
     @objc
     private func applicationDidReceiveMemoryWarning(_ notification: NSNotification) {
-        DispatchQueue.global().async {
-            self.releaseMemory()
+        writer.async { db in
+            db.releaseMemory()
         }
     }
     #endif