Deadlock in VolumeDataRequestProcessor::WaitForCompletion after cancel loading one of LODs

We create several LODs in OpenVDS container. When our viewer display data, some time need to cancel loading one of the LODs. It produce deadlock in open-vds library.

Two threads that are in the deadlock: stack_1.txt and stack_2.txt

To fix that, we do next changes: VolumeDataRequestProcessor.patch

Problem occurred on Linux, open-vds 3.0.3. But we found that in latest version function VolumeDataRequestProcessor::WaitForCompletion did not changed.