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/net/zmq_async.cpp

@@ -86,7 +86,7 @@ namespace net { namespace zmq
     }
   }
 
-  expect<async_client> async_client::make(boost::asio::io_service& io, socket zsock)
+  expect<async_client> async_client::make(boost::asio::io_context& io, socket zsock)
   {
     MONERO_PRECOND(zsock != nullptr);