Revamp the rabit implementation. (#10112)

This PR replaces the original RABIT implementation with a new one, which has already been partially merged into XGBoost. The new one features:
- Federated learning for both CPU and GPU.
- NCCL.
- More data types.
- A unified interface for all the underlying implementations.
- Improved timeout handling for both tracker and workers.
- Exhausted tests with metrics (fixed a couple of bugs along the way).
- A reusable tracker for Python and JVM packages.

tests/cpp/objective/test_objective.cc

@@ -50,7 +50,7 @@ class TestDefaultObjConfig : public ::testing::TestWithParam<std::string> {
 
  public:
   void Run(std::string objective) {
-    auto Xy = MakeFmatForObjTest(objective);
+    auto Xy = MakeFmatForObjTest(objective, 10, 10);
     std::unique_ptr<Learner> learner{Learner::Create({Xy})};
     std::unique_ptr<ObjFunction> objfn{ObjFunction::Create(objective, &ctx_)};