I dont believe that this should be strictly necessary because `retrieveMetaData` is only ever called in `NewDiskQueue` and the `ioLoop` goroutine is launched *after* that call (which according to the go memory model is safe).

However, I'm not 100% sure about interactions between the go memory model, go-routines, and the combined use of atomic and non-atomic operations (which is what this was relying on before this change... i.e. this was the only mutation of `d.depth` that was **not** using atomic ops).