Fix compile error: use diskv.WriteStream, not old diskv.WriteAndSync method.

diskv.WriteAndSync was removed in github.com/peterbourgon/diskv commit
e8bbf390fbf24be1df0f5d62d30591a32ab158c5:
https://github.com/peterbourgon/diskv/commit/e8bbf390fbf24be1df0f5d62d30591a32ab158c5#L2L96
Merge pull request #6 from sourcegraph/diskv-WriteStream-interface

Fix compile error: use diskv.WriteStream, not old diskv.WriteAndSync method