[Client] Fix possible goroutine leak.

Running goroutines with pullLayer are blocked to send error of a
pull operation. If we abort pulling without notify them about
cancelation they will get stucked forever. To avoid this possible
leak cancelCh was introduced. In case of abort we close that channel
to notify other goroutines about cancelation.
Adds a test for concurrent storagedriver Write/Read Stream operations

This test is currently failing and Skipped for IPC drivers
Typo fix
Merge pull request #766 from noxiouz/fix_possible_goroutine_leak

[Client] Fix possible goroutine leak in pull
Merge pull request #767 from BrianBland/ng-storagedriver-concurrency-test

Adds a test for concurrent storagedriver Write/Read Stream operations
Merge pull request #756 from docker/test-drone

Move to own dev image
[Client] Fix possible goroutine leak in push.

The same as 5a804ac05bc3facd61953908da99310a257727a1
Merge pull request #770 from noxiouz/fix_leak_in_push

[Client] Fix possible goroutine leak in push.
Move notifications to dt