Fix completed var thread safety problem

[Mourad-Aly]

Hi,

I've faced racing condition while using Bolts in our Apps, same issue reported before #302
Hot fix is to ensure locking the completed variable, however I don't know why wasn't it already in the lock block (deadlock?)? This is why I'm submitting this PR to ensure that the server tests will not fail since I'm unable to run tests locally (will figure this out).
I'd suggest handling thread safety for BFTask in an easier-to-read way than locks, what do you think? GCD for example, sync_barrier?

Updates: I've just seen #303 👍

[Mourad-Aly]

Hi @nlutsenko,
Seems a deadlock, since self.condition will wait in self.lock, and self.condition's signaling is happening in another self.lock block as well. I'm running tests locally to check whether it's caught by tests or not.
Any concern is appreciated. Thanks.