Rewrite cache loader to async framework (#1193)

Fixes #1188

unittests/util/async/AnyStopTokenTests.cpp

@@ -19,6 +19,8 @@
 
 #include "util/async/AnyStopToken.hpp"
 
+#include <boost/asio/spawn.hpp>
+#include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
 using namespace util::async;
@@ -36,6 +38,7 @@ struct FakeStopToken {
 }  // namespace
 
 struct AnyStopTokenTests : public TestWithParam<bool> {};
+using AnyStopTokenDeathTest = AnyStopTokenTests;
 
 INSTANTIATE_TEST_CASE_P(AnyStopTokenGroup, AnyStopTokenTests, ValuesIn({true, false}), [](auto const& info) {
     return info.param ? "true" : "false";
@@ -57,3 +60,10 @@ TEST_P(AnyStopTokenTests, IsStopRequestedCallPropagated)
     EXPECT_EQ(stopToken.isStopRequested(), flag);
     EXPECT_EQ(stopToken, flag);
 }
+
+TEST_F(AnyStopTokenDeathTest, ConversionToYieldContextAssertsIfUnsupported)
+{
+    EXPECT_DEATH(
+        [[maybe_unused]] auto unused = static_cast<boost::asio::yield_context>(AnyStopToken{FakeStopToken{}}), ".*"
+    );
+}