* Fix crashes caused by failing to unlock or destroy a static mutex while the app is being terminated
* Allocate static mutexes on the heap memory to avoid destruction at app exit
* ASThread to use ASDisplayNodeCAssert() instead of assert()
* [ASMultiplex/NetworkImageNode] Ensure ASImageProtocols are not called with nil arguments.
* [ASBasic/PINRemoteImageDownloader] Do not check for nil downloadIdentifier needlessly.
* [ASImageCacheProtocol] Make synchronouslyFetchedCachedImageWithURL URL nonnull.
