Need to replace thread unsafe code from TextureCache::addImageAsync

[sbrednikhin]

TextureCache::loadImage will be runnedin parallel thread by TextureCache::addImageAsync, but _textures.find() and _textures.end() used inside aren't thread-safe and there is no lock.

http://discuss.cocos2d-x.org/t/looks-like-texturecache-loadimage-is-not-thread-safe/21414

[walzer]

Yes, this is a critical bug!

[0xJimmyBeh]

@walzer
There is a long existing issue with TextureCache.addImageAsync in Cocos2d-JS too.

cocos2d/cocos2d-js#1658

Please solve them together since all engine going to merge soon.

Thanks.