Tentative fix for cancellation in AsyncLock

Before this a cancelled LockAsync call would still report a successful
lock and return the inner Release disposable which in turn could lead to
SemaphoreFullException as a lock not held was being released.

Akavache.Sqlite3/AsyncLock.cs

@@ -19,10 +19,13 @@ public Task<IDisposable> LockAsync(CancellationToken ct = default(CancellationTo
         {
             var wait = m_semaphore.WaitAsync(ct);
 
-            return wait.IsCompleted ?
-                m_releaser :
-                wait.ContinueWith((_, state) => (IDisposable)state,
-                    m_releaser.Result, CancellationToken.None,
+            // Happy path. We synchronously acquired the lock.
+            if (wait.IsCompleted && !wait.IsFaulted)
+                return m_releaser;
+
+            return wait
+                .ContinueWith((_, state) => (IDisposable)state,
+                    m_releaser.Result, ct,
                     TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
         }