Update the .seen interval and optionally a source specific seen node.

async def seen(self, tick, source=None):
        '''
        Update the .seen interval and optionally a source specific seen node.
        '''
        await self.set('.seen', tick)

        if source is not None:
            seen = await self.snap.addNode('meta:seen', (source, self.ndef))
            await seen.set('.seen', tick)