Revert LATENCY_HDR_SEC_SIGDIGTS to 2. Optimize summarize_current_second by using hdr_value_at_percentiles() instead of N x hdr_value_at_percentile().

Revert LATENCY_HDR_SEC_SIGDIGTS to 2. Optimize summarize_current_second by using hdr_value_at_percentiles() instead of N x hdr_value_at_percentile(). #572