Removed CLOCK_MONOTONIC_COARSE support. While clock_gettime(CLOCK_MONOTONIC_COARSE) is faster than clock_gettime(CLOCK_MONOTONIC), the latter is fast enough on Linux for practical usage, and the difference is negligible compared to other costs at each event loop iteration. On the other hand, CLOCK_MONOTONIC_COARSE causes various issues with typical CONFIG_HZ=250, notably very inaccurate limit_rate handling in some edge cases (ticket #1678) and negative difference between $request_time and $upstream_response_time (ticket #1965).