crimson/os/seastore: make segment cleaner do gc based on gc benefit cost ratio

crimson/os/seastore: make segment cleaner do gc based on gc benefit cost ratio

Segments' benefit cost ratio is calculated:
segment_last_mod_time = max{last modification time of all extents in the segment}
segment_last_rewrite_time = max{last rewrite time of all extents in the segment}
segment_utilization = segment_live_bytes / segment_size
segment_age = current_time - max{segment_last_mod_time, segment_last_rewrite_time}
benefit_cost_ratio = (1 - segment_utilization) * segment_age / (1 + segment_utilization)

Segment cleaner always reclaim the segment with the largest benefit cost ratio

Signed-off-by: Xuehan Xu <xxhdx1985126@gmail.com>