Atomically increments a node's value.

async def add(self, full, valu):
        '''
        Atomically increments a node's value.
        '''
        node = await self.open(full)

        oldv = node.valu
        newv = oldv + valu

        node.valu = await self.storNodeValu(full, node.valu + valu)

        await node.fire('hive:set', path=full, valu=valu, oldv=oldv)

        return newv