cachedblobstore: Stale writeback may fail to query version

[nyaxt]

In unfortunate case, a cached blob writeback may initiate when frontend is just updating its chunk header.
cachedblobentry.writeBackWithLock will fail in that case:

2016/01/19 22:57:55 W [cachedbs] cachesyncer.go:101: Worker[0] Sync failed CachedBlobEntry{"7a227c943c8c10340984094988b8ae4d", Dirty} err: Failed to writeback dirty: Failed to query cached blob ver: signature magic mismatch

[nyaxt]

Labeling as P2, as it shouldn't cause any real issue. Next Sync() should succeed fine.