The more I think about it the more wrong it feels to have Simple_store know about Quiesce.Mode.t.
The way I see it quiescence is a property of the state machine (which also happens to be enforced in the store, to protect ourself against bugs). Quiescence can happen with a read_only or a read_write store.
Simple_store to me is just a key-value database which happens to be opened for reading or for writing, it doesn't know about the state machine.
Long story short, I think I prefer just having the read_only boolean flag here ..
(I know you asked a question about Quiesce mode being the right approach, but in hindsight I didn't think it through and only realized my preference while reading this pull request.)