MB-15755 Use nil ts to represent storage state after flush
MB-15755 Use nil ts to represent storage state after flush

After bucket flush, the storage gets rolled back to zero.
Use nil ts to represent this rather than a zero ts. A nil
ts correctly denotes that no flush has happened yet for the bucket.

Change-Id: I9ae3e0487be49e919c7444b057944e442667a3cc

jchris/indexing,jchris/indexing,jchris/indexing,jchris/indexing