Handling of IO errors

Currently any IO errors are going to cause a panic, for the most part since our trait design evolved around in-memory only. Now this is fine for the most part, most libraries like rocksdb do this internally, however, for custom code we require diligence to not i.e. panic on an `EAGAIN` which is non-fatal (not specific to `SmtStore`). These are two separate concerns, yet they're closely related - error handling of IO related errors.