Iterate (buid, valu) rows for the given form:prop in this layer.

async def iterPropRows(self, form, prop):
        '''
        Iterate (buid, valu) rows for the given form:prop in this layer.
        '''
        # iterate byprop and join bybuid to get to value

        penc = prop.encode()
        pref = form.encode() + b'\x00' + penc + b'\x00'

        for _, pval in self.layrslab.scanByPref(pref, db=self.byprop):

            buid = s_msgpack.un(pval)[0]

            byts = self.layrslab.get(buid + penc, db=self.bybuid)
            if byts is None:
                continue

            valu, indx = s_msgpack.un(byts)

            yield buid, valu