eth_getLogs on BSC: a refused block range looks exactly like a quiet pool — tell them apart, and measure the window in time

Reading Swap or Transfer logs to measure a pool or a token, we printed 'no trades' about pools that had trades: the node had refused the range, and an empty answer and a refusal were handled alike. Keep them apart: an error is 'not read', [] is 'nothing happened'. Two more: - Measure a window from block timestamps, not from an assumed block time. In September 5,000 BSC blocks were about 38 minutes, not the two hours an old 3-second assumption gives; block times change with upgrades. - Ranges further back can be 'archive' on public nodes and are refused — a second, older window of ours was refused every time, so we dropped it rather than report a hole as calm. Also take an event's topic from the logs of a live pool, not from documentation; and public nodes rate-limit batches (#7).