fix: Data race in new webserver (#1926)

There was a data race inside `CoroutineGroup` because internal timer was
used from multiple threads in the methods `asyncWait()` and
`onCoroutineComplete()`. Changing `registerForeign()` to spawn to the
same `yield_context` fixes the problem because now the timer is accessed
only from the same coroutine which has an internal strand.

During debugging I also added websocket support for `request_gun` tool.

src/util/CoroutineGroup.cpp

@@ -56,13 +56,15 @@ CoroutineGroup::spawn(boost::asio::yield_context yield, std::function<void(boost
 }
 
 std::optional<std::function<void()>>
-CoroutineGroup::registerForeign()
+CoroutineGroup::registerForeign(boost::asio::yield_context yield)
 {
     if (isFull())
         return std::nullopt;
 
     ++childrenCounter_;
-    return [this]() { onCoroutineCompleted(); };
+    // It is important to spawn onCoroutineCompleted() to the same coroutine as will be calling asyncWait().
+    // timer_ here is not thread safe, so without spawn there could be a data race.
+    return [this, yield]() { boost::asio::spawn(yield, [this](auto&&) { onCoroutineCompleted(); }); };
 }
 
 void