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/ci_build/Dockerfile.jvm_cross

@@ -1,6 +1,6 @@
 FROM ubuntu:18.04
 ARG JDK_VERSION=8
-ARG SPARK_VERSION=3.0.0
+ARG SPARK_VERSION=3.4.0
 
 # Environment
 ENV DEBIAN_FRONTEND noninteractive