Andrew Savonichev authored Aug 18, 2016 and Dmitry Kazakov committed Aug 18, 2016

Summary:
KisUpdaterContext::waitForDone() method must lock context and wait
untill all threads finish their current tasks.
We cannot call QThreadPool.waitForDone() to do this, because after
waiting it removes all threads from the pool.

Test Plan:
Verified that no extra threads created: 8 for global thread pool and 8 for
KisUpdaterContext

(gdb) info threads
  Id   Target Id         Frame
  22   Thread 0x7fff8cff9700 (LWP 18481) "Thread (pooled)" 0x00007fffef6e2cc8 in pthread_cond_timedwait () from /lib64/libpthread.so.0
  21   Thread 0x7fff8d7fa700 (LWP 18480) "Thread (pooled)" 0x00007fffef6e2cc8 in pthread_cond_timedwait () from /lib64/libpthread.so.0
  20   Thread 0x7fff8dffb700 (LWP 18479) "Thread (pooled)" 0x00007fffef6e2cc8 in pthread_cond_timedwait () from /lib64/libpthread.so.0
  19   Thread 0x7fff8e7fc700 (LWP 18478) "Thread (pooled)" 0x00007fffef6e2cc8 in pthread_cond_timedwait () from /lib64/libpthread.so.0
  18   Thread 0x7f...