[coll] Implement a new tracker and a communicator. (#9650)

* [coll] Implement a new tracker and a communicator.

The new tracker and communicators communicate through the use of JSON documents. Along
with which, communicators are aware of each other.

tests/cpp/collective/test_comm.cc

@@ -0,0 +1,47 @@
+/**
+ * Copyright 2023, XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+
+#include "../../../src/collective/comm.h"
+#include "test_worker.h"
+namespace xgboost::collective {
+namespace {
+class CommTest : public TrackerTest {};
+}  // namespace
+
+TEST_F(CommTest, Channel) {
+  auto n_workers = 4;
+  RabitTracker tracker{host, n_workers, 0, timeout};
+  auto fut = tracker.Run();
+
+  std::vector<std::thread> workers;
+  std::int32_t port = tracker.Port();
+
+  for (std::int32_t i = 0; i < n_workers; ++i) {
+    workers.emplace_back([=] {
+      WorkerForTest worker{host, port, timeout, n_workers, i};
+      if (i % 2 == 0) {
+        auto p_chan = worker.Comm().Chan(i + 1);
+        p_chan->SendAll(
+            EraseType(common::Span<std::int32_t const>{&i, static_cast<std::size_t>(1)}));
+        auto rc = p_chan->Block();
+        ASSERT_TRUE(rc.OK()) << rc.Report();
+      } else {
+        auto p_chan = worker.Comm().Chan(i - 1);
+        std::int32_t r{-1};
+        p_chan->RecvAll(EraseType(common::Span<std::int32_t>{&r, static_cast<std::size_t>(1)}));
+        auto rc = p_chan->Block();
+        ASSERT_TRUE(rc.OK()) << rc.Report();
+        ASSERT_EQ(r, i - 1);
+      }
+    });
+  }
+
+  for (auto &w : workers) {
+    w.join();
+  }
+
+  ASSERT_TRUE(fut.get().OK());
+}
+}  // namespace xgboost::collective