Update boost::asio usage to conform to newer standards: (#144)

* Convert boost::asio::io_service to boost::asio::io_context
  * Convert strand.wrap(...) to boost::asio::bind_executor(strand, ...)
  * Convert strand.dispatch(...) to boost::asio::dispatch(strand, ...)
  * Convert io_context.reset() to io_context.restart()
  * Convert null_buffers() usage to socket.async_wait(...)
  * Drop usage of GET_IO_SERVICE macro from monero
  * Refactor REST server to manage resources better

src/client_main.cpp

@@ -294,7 +294,7 @@ namespace
         boost::this_thread::sleep_for(boost::chrono::seconds{1});
 
       self.stop_ = false;
-      self.io_.reset();
+      self.io_.restart();
       if (self.has_shutdown())
         return;