Issue 5183 - Provide async interface for FileSystem

src/JsEngine.cpp

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2017 eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 311 matching lines @@
 AdblockPlus::FileSystemPtr AdblockPlus::JsEngine::GetAsyncFileSystem() const
 {
   return fileSystem;
 }
 
 void AdblockPlus::JsEngine::SetFileSystem(const AdblockPlus::FileSystemSyncPtr& val)
 {
   if (!val)
     throw std::runtime_error("FileSystem cannot be null");
 
-  fileSystemLegacy = val;

[sergei, 2017/07/05 10:03:21]

BTW, this `fileSystemLegacy` member is not used, so if the user sets it it will
have no effect. It seems the best option is to create async implementation with
injected `val`.

[hub, 2017/07/06 12:21:52]

We don't have a visibility of what filesystem has been created so we can't
simply recreate it. 

The few alternative I see are:
1. Add a method to IFileSystem to do this. We might need to address a few issues
in our own test though.
2. remove this method, but this would break API

One of these should happen.

[sergei, 2017/07/06 14:14:04]

I thought about removing of legacy member and setting 
fileSystem.reset(new DefaultFileSystem(val));

[hub, 2017/07/06 21:24:29]

Done.

+  fileSystem.reset(new DefaultFileSystem(val));
 }
 
 void AdblockPlus::JsEngine::SetWebRequest(const AdblockPlus::WebRequestSharedPtr& val)
 {
   if (!val)
     throw std::runtime_error("WebRequest cannot be null");
 
   webRequestLegacy = val;
 }
 
@@ skipping 10 matching lines @@
   logSystem = val;
 }
 
 
 void AdblockPlus::JsEngine::SetGlobalProperty(const std::string& name,
                                               const AdblockPlus::JsValue& value)
 {
   auto global = GetGlobalObject();
   global.SetProperty(name, value);
 }