Support multithreaded downloads for RemoteData

All posters for a search selection are downloaded before the first one
is shown in the UI.  For popular tv series, there can be 20 or more posters
for the series which are all downloaded before the 'first' poster appears in the UI.
Currently, the posters are downloaded serially which means that on a slow-ish
connection to tvDB (for eg) this can take quite a while before the 'first' poster
appears in the UI.

This patch moves patch downloading onto the global 'utility' thread which allows
for multiple simultaneous downloads, limiting the number of simultaneous
downloads to 10.

Framework/src/RemoteData.swift

@@ -7,28 +7,27 @@
 
 import Foundation
 
-fileprivate class RemoteCaching {
-    // Used a session that caches its results
-    static let remoteLoadSession: URLSession = {
-        // Create URL Session Configuration
-        let configuration = URLSessionConfiguration.default.copy() as! URLSessionConfiguration
+// Limit the number of simultaneous downloads
+private let RemoteDownloadLimiter = DispatchSemaphore(value: 10)
 
-        // Set the in-memory cache to 128 MB
-        let cache = URLCache()
-        cache.memoryCapacity = 128 * 1024 * 1024
-        configuration.urlCache = cache
+// A URLSession that caches its results
+private let RemoteLoadSession: URLSession = {
+    // Create URL Session Configuration
+    let configuration = URLSessionConfiguration.default.copy() as! URLSessionConfiguration
 
-        // Define Request Cache Policy
-        configuration.requestCachePolicy = .useProtocolCachePolicy
-        configuration.urlCache = cache
+    // Set the in-memory cache to 128 MB
+    let cache = URLCache()
+    cache.memoryCapacity = 128 * 1024 * 1024
+    configuration.urlCache = cache
 
-        return URLSession(configuration: configuration)
-    }()
-}
+    // Define Request Cache Policy
+    configuration.requestCachePolicy = .useProtocolCachePolicy
+    configuration.urlCache = cache
+
+    return URLSession(configuration: configuration)
+}()
 
 @objc public class RemoteData : NSObject {
-    private static let queue = DispatchQueue(label: "io.metaz.RemoteDataQueue")
-
     public let url : URL
     public let expectedMimeType : String
 
@@ -110,13 +109,19 @@ fileprivate class RemoteCaching {
 
             let url = self.data!.url
             let expectedMimeType = self.data!.expectedMimeType
-            RemoteData.queue.async {
+
+            DispatchQueue.global(qos: .utility).async {
+                RemoteDownloadLimiter.wait()
+                defer {
+                    RemoteDownloadLimiter.signal()
+                }
+
                 var downloadData : Data?, responseError : NSError?
                 let signal = DispatchSemaphore(value: 0)
                 if self.data == nil {
                     return;
                 }
-                RemoteCaching.remoteLoadSession.dataTask(with: url) { (d, resp, err) in
+                RemoteLoadSession.dataTask(with: url) { (d, resp, err) in
                     if let error = err {
                         let info = [NSLocalizedDescriptionKey: error.localizedDescription]
                         let statusCode = (resp as? HTTPURLResponse)?.statusCode ?? 0