Fixed deadlock when executing fetch request

[nextgeniuspro]

Fixed deadlock when executing fetch request from main and background threads simultaneously. As Apple docs say about using performBlockAndWait: "The one exception is: if your code is executing on the main thread, you can invoke methods on the main queue style contexts directly instead of using the block based API."

[nextgeniuspro]

Strange that CI finished building with failure status. These changes cannot affect on compilation result

[ssuhanov]

[Coeur]

@yevgeniy-logachev how to reproduce the deadlock ? can you demonstrate it with a piece of code?

[Coeur]

@yevgeniy-logachev could you at least rebase your branch to the current tip to see if the Travis builds can pass?