Return stream of (iden, provenance stack) tuples at the given offset.

async def provStacks(self, offs, size):
        '''
        Return stream of (iden, provenance stack) tuples at the given offset.
        '''
        count = 0
        for iden, stack in self.cell.provstor.provStacks(offs, size):
            count += 1
            if not count % 1000:
                await asyncio.sleep(0)
            yield s_common.ehex(iden), stack