mailbox: Extract constants from configuration keys
mailbox: Extract constants from configuration keys

This refactoring makes finding where a configuration value is used
eaiser.

doortts/forked-for-history,oolso/yobi,ahb0327/yobi,doortts/forked-for-history,yona-projects/yona,brainagenet/yobi,oolso/yobi,Limseunghwan/oss,brainagenet/yobi,doortts/forked-for-history,yona-projects/yona,bloodybear/yona,brainagenet/yobi,naver/yobi,ahb0327/yobi,Limseunghwan/oss,doortts/fork-yobi,ihoneymon/yobi,ihoneymon/yobi,oolso/yobi,bloodybear/yona,bloodybear/yona,naver/yobi,ahb0327/yobi,ihoneymon/yobi,Limseunghwan/oss,doortts/fork-yobi,naver/yobi,doortts/fork-yobi,doortts/fork-yobi,yona-projects/yona,bloodybear/yona,yona-projects/yona