Add waitForAllTaskTermination for fixing segmentation fault of Carthage

[norio-nomura]

This is rebased version of #21.

Fix segmentation fault on termination of Carthage.

Segmentation fault on termination of Carthage 0.7.4 beta-2 happens following scenario:

1. launchTask's thread call sendCompleted() and kick main thread
   2a. main thread wake up and call exit()
   2b. launchTask's thread continue disposing
2. main thread cause segmentation fault.

Waiting completed from launchTask's thread is not enough for fixing the segmentation fault.
It needs waiting actual termination of launchTask's thread

Carthage/Carthage#474 (comment)

[robrix]

It looks like @jspahrsummers left some feedback on #21. Could you that feedback on this PR please?

[norio-nomura]

Thanks for reviewing.
As @jspahrsummers wrote on #21 (comment), his comments on lines are not valuable for the purpose of this PR.
He implemented #23 from my suggestion by #21, using fine-grained and waiting per-task logic using RAC way. But #23 did not be replacement of #21.
So, I push this PR.

[robrix]

Okay, thanks for the info. I think given the practical considerations that we ought to merge this PR, and then if it needs adjustment we can do that later on.

Thanks very much for the PR and your investigation into the segfaults, @norio-nomura!

[norio-nomura]

Thank you for reviewing & merging it.

I have one more ReactiveCocoa/ReactiveCocoa#2058 related to Carthage hangs. I wish it will be also merged to next release.

[robrix]

Cool, merged.

[norio-nomura]

Thanks!

[norio-nomura]

Oh, I should said about ReactiveCocoa before I pushed update Cartfile, Sorry. 😔

[robrix]

Don’t sweat it!

[norio-nomura]

Thanks,
I don't have any pending PRs about Carthage.