I like the idea of consolidating file descriptors so I added an issue: https://github.com/boltdb/bolt/issues/106

Mocking the file via an interface seems a little bit like overkill. I don't want to add an external API just for the sake of testing. Maybe an alternate, unexported open function could work:

```go
func open(path string, mode os.FileMode, ops) (*DB, error)
```