Update the default for compaction_throughput_mb_per_sec

compaction_throughput_mb_per_sec has been at 16 since probably 0.6 or 0.7 back when a lot of people had to deploy on spinning disks. It seems like it would make sense to update the default to something more reasonable - assuming a reasonably decent SSD and competing IO. One idea that could be bikeshedded to death could be to just default it to 64 - simply to avoid people from having to always change that any time they download a new version as well as avoid problems with new users thinking that the defaults are sane.