mu-server: implement temp-file optimization

It can be faster to feed big mu -> mu4e data, such as contacts are
message headers through a temp-file instead directly though stdout;
implement this, and add the server parameter --allow-temp-file.

Implement this the "contacts" and "find" commands.

mu/mu-options.cc

@@ -498,6 +498,10 @@ sub_server(CLI::App& sub, Options& opts)
 	sub.add_option("--eval", opts.server.eval,
 		       "Evaluate mu server expression")
 		->excludes("--commands");
+	sub.add_flag("--allow-temp-file", opts.server.allow_temp_file,
+		     "Allow for the temp-file optimization")
+		->excludes("--commands");
+
 }
 
 static void