Mask no-data pause-point, add perps to sig.

Was orig for debugging an issue with `kucoin` i think but definitely
shouldn't be left in XD

Also add `'perpetual_future'` to `start_backfill()` input literal set.

piker/tsp/__init__.py

@@ -458,13 +458,15 @@ async def start_backfill(
                     'bf_until <- last_start_dt:\n'
                     f'{backfill_until_dt} <- {last_start_dt}\n'
                 )
-
-                # ugh, what's a better way?
-                # TODO: fwiw, we probably want a way to signal a throttle
-                # condition (eg. with ib) so that we can halt the
-                # request loop until the condition is resolved?
-                if timeframe > 1:
-                    await tractor.pause()
+                # UGH: what's a better way?
+                # TODO: backends are responsible for being correct on
+                # this right!?
+                # -[ ] in the `ib` case we could maybe offer some way
+                #     to halt the request loop until the condition is
+                #     resolved or should the backend be entirely in
+                #     charge of solving such faults? yes, right?
+                # if timeframe > 1:
+                #     await tractor.pause()
                 return
 
             assert (
@@ -578,6 +580,7 @@ async def start_backfill(
                     'crypto',
                     'crypto_currency',
                     'fiat',  # a "forex pair"
+                    'perpetual_future',  # stupid "perps" from cex land
                 }:
                     # for now, our table key schema is not including
                     # the dst[/src] source asset token.