Move the handle idle command into the command queue

My theory is that it's ok to send the `noidle` synchronously but the
subsequent handling of the idle response should be on the command queue.
My hope is that this fixes issues with less than perfect network
connections (and also very occasionally seen on good networks).

Shared/MPDClient/Extensions/MPDClient+Idle.swift

@@ -13,49 +13,36 @@ extension MPDClient {
   func noIdle() {
     guard isConnected else { return }
     
-    do {
-      idleLock.lock()
-      defer { idleLock.unlock() }
     if isIdle {
       mpd_send_noidle(connection)
-        isIdle = false
-      }
     }
   }
 
   func idle(_ force: Bool = false) {
     guard isConnected else { return }
 
-    let shouldIdle: Bool
+    let shouldIdle: Bool = (!isIdle && commandQueue.operationCount == 1) || force
     
-    do {
-      idleLock.lock()
-      defer { idleLock.unlock() }
-      shouldIdle = (!isIdle && commandQueue.operationCount == 1) || force
     if shouldIdle {
-        mpd_send_idle(connection)
       self.isIdle = true
+      mpd_send_idle(connection)
+      
+      let handleIdleOperation = BlockOperation() { [unowned self] in
+        self.handleIdleResult()
+      }
+      
+      handleIdleOperation.queuePriority = .high
+      commandQueue.addOperation(handleIdleOperation)
     }
   }
 
-    if shouldIdle {
+  func handleIdleResult() {
     let result = mpd_recv_idle(connection, true)
-      handleIdleResult(result)
-    }
-  }
-
-  func handleIdleResult(_ result: mpd_idle) {
     let mpdIdle = MPDIdle(rawValue: result.rawValue)
-    let wasIdle: Bool
     
-    do {
-      idleLock.lock()
-      defer { idleLock.unlock() }
-      wasIdle = isIdle
     isIdle = false
-    }
 
-    if checkError() && wasIdle {
+    if checkError() {
       if mpdIdle.contains(.database) {
         self.fetchAllAlbums()
       }

Shared/MPDClient/MPDClient.swift

@@ -22,8 +22,6 @@ class MPDClient {
 
   let commandQueue = OperationQueue()
 
-  let idleLock = NSLock()
-
   init(withDelegate delegate: MPDClientDelegate?) {
     commandQueue.maxConcurrentOperationCount = 1
     self.delegate = delegate