sui_v1.78.0_1786670643_ci: Tweak bigtable write controller (#27690)

Description Last night I got paged because the mainnet json-rpc fallback bigtable cluster was lagging. A brief, large but short-lived latency spike caused the write controller to throttle writes down to 0.1 QPS and the controller got stuck there and I had to restart the indexers manually to recover. The controller watches flow control feedback from bigtable that tells it by what factor it can…