Enable multi-threading for MSCCL (#1203)

MSCCL can now run in a multi-threaded configuration. To test in the unit tests, added the ENABLE_OPENMP compile definition flag and the --openmp-test-enable flag to the unit test build script. To activate, set the environment variables UT_MULTITHREADED=1 and UT_PROCESS_MASK=1. Set Jenkins to use this mode.

test/common/TestBed.cpp

@@ -97,7 +97,7 @@ namespace RcclUnitTesting
     childList.resize(this->numActiveChildren);
     for (int childId = 0; childId < this->numActiveChildren; ++childId)
     {
-      childList[childId] = new TestBedChild(childId, ev.verbose, ev.printValues);
+      childList[childId] = new TestBedChild(childId, ev.verbose, ev.printValues, ev.useMultithreading);
       if (childList[childId]->InitPipes() != TEST_SUCCESS)
       {
         ERROR("Unable to create pipes to child process\n");