Update closure reference

[onmyway133]

• There are some errors like incorrect checksum for freed object - object was probably modified after being freed that may relate to inner closure reference.

  • https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/001197.html
  • http://blog.xebia.com/swift-self-reference-in-inner-closure/

From testing https://gist.github.com/onmyway133/0c76f56c52e4c3a4c0d705481dfaacbc, I see that there's nothing wrong. But I think it's easy to reason with another helper function

• Running Thead Sanitizer, we have some warnings about accessing fetcher from main queue and from cache queue. This fixes by wrapping the call inside main queue so that actions are processed serially

[mention-bot]

@onmyway133, thanks for your PR! By analyzing the history of the files in this pull request, we identified @zenangst, @vadymmarkov and @RamonGilabert to be potential reviewers.

[zenangst]

Very nice find @onmyway133 !

[onmyway133]

What do you think @hyperoslo/ios

[zenangst]

@onmyway133 🚀