Minimize file system requests during image cache lookup

Tusker/Caching/Cache.swift

@@ -15,6 +15,7 @@ enum Cache<T> {
     case disk(DiskStorage<T>)
     case hybrid(HybridStorage<T>)
     
+    @available(*, deprecated, message: "disk-based caches synchronously interact with the file system. Avoid using if possible.")
     func existsObject(forKey key: String) throws -> Bool {
         switch self {
         case let .memory(memory):

Tusker/Caching/ImageCache.swift

@@ -38,8 +38,7 @@ class ImageCache {
     
     func get(_ url: URL, completion: ((Data?) -> Void)?) -> Request? {
         let key = url.absoluteString
-        if (try? cache.existsObject(forKey: key)) ?? false,
-            let data = try? cache.object(forKey: key) {
+        if let data = try? cache.object(forKey: key) {
             completion?(data)
             return nil
         } else {