[rabit_bootstrap_cache ] failed xgb worker recover from other workers (#4808)

* Better recovery support.  Restarting only the failed workers.

tests/ci_build/build_mock_cmake.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -e
+
+rm -rf build
+mkdir build
+cd build
+cmake -DRABIT_MOCK=ON -DCMAKE_VERBOSE_MAKEFILE=ON ..
+make clean
+make -j$(nproc)
+cd ..