Core/Network: Refactor AsyncAcceptor to use async_accept overload producing sockets through argument instead of having to preallocate it

* Also improve main() cleanup to fully process all queued async operations (including their cancellations)

src/common/Threading/ThreadPool.h

@@ -27,6 +27,8 @@ namespace Trinity
 class ThreadPool
 {
 public:
+    using executor_type = boost::asio::thread_pool::executor_type;
+
     explicit ThreadPool(std::size_t numThreads = std::thread::hardware_concurrency()) : _impl(numThreads) { }
 
     template<typename T>