Sort recent permanodes first by time, then by blobref.

Just in case two permanodes were modified in the same nanosecond (or
more likely: the client's clock resolution sucks when they created the
mutation).

Fixes http://camlistore.org/issue/261

Change-Id: I293fc0f771bccc0473fefdcc2178284066b7a05c
Merge branch 'master' of https://camlistore.googlesource.com/camlistore