[offline] Fix race condition in instantiation of local index

It turns out that [lazy variables are not thread-safe](http://stackoverflow.com/a/29761777). We therefore have to synchronize initialization:

- For `OfflineIndex`, we can instantiate the local index during init.
- For `MirroredIndex`, we still need lazy instantiation, so we synchronize access explicitly.

Author: Clément Le Provost

Source/Helpers.swift

@@ -120,6 +120,23 @@ extension Date {
     }
 }
 
+// MARK: - Thread synchronization
+
+extension NSObject {
+    /// Equivalent of Objective-C's `@synchronized` statement.
+    /// Actually leverages the same Objective-C runtime feature (this is why it's only available on `NSObject`).
+    ///
+    /// - paremeter block: Block to be executed serially on the object.
+    ///
+    func synchronized(_ block: () -> ()) {
+        objc_sync_enter(self)
+        defer {
+            objc_sync_exit(self)
+        }
+        block()
+    }
+}
+
 // MARK: - Miscellaneous
 
 /// The operating system's name.

Source/Offline/MirroredIndex.swift

@@ -135,7 +135,7 @@ import Foundation
     }
 
     /// The local index mirroring this remote index (lazy instantiated, only if mirroring is activated).
-    lazy var localIndex: LocalIndex = LocalIndex(dataDir: self.offlineClient.rootDataDir, appID: self.client.appID, indexName: self.name)
+    var localIndex: LocalIndex!
 
     /// The mirrored index settings.
     let mirrorSettings = MirrorSettings()
@@ -146,10 +146,21 @@ import Foundation
             if (mirrored) {
                 do {
                     try FileManager.default.createDirectory(atPath: self.indexDataDir, withIntermediateDirectories: true, attributes: nil)
+                    // Lazy instantiate the local index.
+                    self.synchronized {
+                        if (self.localIndex == nil) {
+                            self.localIndex = LocalIndex(dataDir: self.offlineClient.rootDataDir, appID: self.client.appID, indexName: self.name)
+                        }
+                    }
                 } catch _ {
                     // Ignore
                 }
                 mirrorSettings.load(self.mirrorSettingsFilePath)
+            } else {
+                // Release the local index.
+                self.synchronized {
+                    self.localIndex = nil
+                }
             }
         }
     }

Source/Offline/OfflineIndex.swift

@@ -132,7 +132,7 @@ public struct IOError: CustomNSError {
     @objc public let client: OfflineClient
 
     /// The local index (lazy instantiated).
-    lazy var localIndex: LocalIndex = LocalIndex(dataDir: self.client.rootDataDir, appID: self.client.appID, indexName: self.name)
+    var localIndex: LocalIndex
 
     /// Queue used to run transaction bodies (but not the build).
     private let transactionQueue: OperationQueue
@@ -155,6 +155,7 @@ public struct IOError: CustomNSError {
         self.name = name
         self.transactionQueue = OperationQueue()
         self.transactionQueue.maxConcurrentOperationCount = 1
+        self.localIndex = LocalIndex(dataDir: self.client.rootDataDir, appID: self.client.appID, indexName: self.name)
     }
 
     override public var description: String {