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/scanner.h

@@ -27,7 +27,7 @@
 #pragma once
 
 #include <atomic>
-#include <boost/asio/io_service.hpp>
+#include <boost/asio/io_context.hpp>
 #include <boost/asio/signal_set.hpp>
 #include <boost/optional/optional.hpp>
 #include <cstdint>
@@ -77,7 +77,7 @@ namespace lws
 
   struct scanner_sync
   {
-    boost::asio::io_service io_;
+    boost::asio::io_context io_;
     std::atomic<bool> stop_;     //!< Stop scanning but do not shutdown
     std::atomic<bool> shutdown_; //!< Exit scanner::run