`binance`: always request an extra 1min OHLC bar

Seems that by default their history indexing rounds down/back to the
previous time step, so make sure we add a minute inside `Client.bars()`
when the `end_dt=None`, indicating "get the latest bar". Add
a breakpoint block that should trigger whenever the latest bar vs. the
latest epoch time is mismatched; we'll remove this after some testing
verifying the history bars issue is resolved.

Further this drops the legacy `backfill_bars()` endpoint which has been
deprecated and unused for a while.

piker/fsp/_engine.py

@@ -439,7 +439,7 @@ async def cascade(
                     profiler.finish()
 
                     async for i in istream:
-                        # log.runtime(f'FSP incrementing {i}')
+                        # print(f'FSP incrementing {i}')
 
                         # respawn the compute task if the source
                         # array has been updated such that we compute