In WT-4561 and WT-4577 there was discussion and issues regarding the hot backup lock and other locks. michael.cahill had a good suggestion:
It seems like we could use a WT_WITH_HOTBACKUP_LOCK macro (along with a WT_SESSION_HOTBACKUP_LOCKED constant) that enforces this lock ordering the way we do for other locks?
We should look into adding/using such a macro.