Not having a skew caused an issue where:
1. Uplink calls "begin segment", where segment isn't committed to the
database.
2. Uplink stores piece X to the storage node A with timestamp 1.
3. Satellite runs garbage collection with timestamp 2.
4. Satellite sends retain request to storage node A with timestamp 2.
5. Storage node A deletes piece X, because 1 < 2.
6. Uplink calls "commit segment" with storage node A in it.
7. Download of segment fails, because A doesn't have piece X.
In production this is not an issue since the MaxTimeSkew is 72h by
default.
Change-Id: Id87ca3ddc44103dcd85d031b1367168c014b8e7b
10d932fd65