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);
 

src/net/zmq_async.h

@@ -28,7 +28,7 @@
 #pragma once
 
 #include <boost/asio/compose.hpp>
-#include <boost/asio/io_service.hpp>
+#include <boost/asio/io_context.hpp>
 #include <boost/asio/posix/stream_descriptor.hpp>
 #include <boost/system/error_code.hpp>
 #include <cstddef>
@@ -63,7 +63,7 @@ namespace net { namespace zmq
     asocket asock;
     bool close;
 
-    static expect<async_client> make(boost::asio::io_service& io, socket zsock);
+    static expect<async_client> make(boost::asio::io_context& io, socket zsock);
   };
 
   class read_msg_op
@@ -94,7 +94,7 @@ namespace net { namespace zmq
           return self.complete(make_error_code(msg.error()), 0);
 
         // try again
-        sock_->asock->async_read_some(boost::asio::null_buffers(), std::move(self));
+        sock_->asock->async_wait(boost::asio::posix::stream_descriptor::wait_read, std::move(self));
         return;
       }
 
@@ -133,7 +133,7 @@ namespace net { namespace zmq
           return self.complete(make_error_code(status.error()), 0);
 
         // try again
-        sock_->asock->async_write_some(boost::asio::null_buffers(), std::move(self));
+        sock_->asock->async_wait(boost::asio::posix::stream_descriptor::wait_write, std::move(self));
         return;
       }