in order to know which blob is 'real' (has content)
we need to know which was the first blob inserted in cache,
because that is always the real one.
because we can not modify the keys order in boltdb we'll do this
by marking the first blob inserted with a value
when GetBlob() return the blob which is marked
when PutBlob() if is the first one, mark it
when DeleteBlob() in case deleted is marked then mark the next blob
Signed-off-by: Petu Eusebiu <peusebiu@cisco.com>
