Fix code analysis warnings (#527)

Author: bitfaster

BitFaster.Caching/Lfu/ConcurrentLfu.cs

@@ -78,7 +78,7 @@ private void DrainBuffers()
         public Optional<ICacheMetrics> Metrics => core.Metrics;
 
         ///<inheritdoc/>
-        public Optional<ICacheEvents<K, V>> Events => core.Events;
+        public Optional<ICacheEvents<K, V>> Events => Optional<ICacheEvents<K, V>>.None();
 
         ///<inheritdoc/>
         public CachePolicy Policy => core.Policy;

BitFaster.Caching/Lfu/ConcurrentLfuCore.cs

@@ -107,8 +107,6 @@ public ConcurrentLfuCore(int concurrencyLevel, int capacity, IScheduler schedule
 
         public Optional<ICacheMetrics> Metrics => new(this.metrics);
 
-        public Optional<ICacheEvents<K, V>> Events => Optional<ICacheEvents<K, V>>.None();
-
         public CachePolicy Policy => new(new Optional<IBoundedPolicy>(this), Optional<ITimePolicy>.None());
 
         public ICollection<K> Keys => this.dictionary.Keys;

BitFaster.Caching/Scheduler/BackgroundThreadScheduler.cs

@@ -35,7 +35,7 @@ public sealed class BackgroundThreadScheduler : IScheduler, IDisposable
         public BackgroundThreadScheduler()
         {
             // dedicated thread
-            Task.Factory.StartNew(() => Background(), cts.Token, TaskCreationOptions.LongRunning, TaskScheduler.Default);
+            _ = Task.Factory.StartNew(async () => await Background().ConfigureAwait(false), cts.Token, TaskCreationOptions.LongRunning, TaskScheduler.Default);
         }
 
         ///<inheritdoc/>