Improve runtime profile counters for slow IO from remote stores

Remote storage systems (e.g. cloud stores like S3 and ABFS) often have long tail latencies. Most I/O finishes relatively quickly, but some calls make take significantly longer. Even for HDFS, this is an issue (e.g. hedged reads were developed to help mitigate tail latencies, although no such feature exists for cloud storage connectors).

Currently, scan nodes just track the total amount of time spent reading data. It would be good to have a summary stats counter that tracks the min, avg, and max time spent reading data. This should at least allow us to identify when calls to remote storage services are taking longer than usual.