Why is corestoraged preventing sleep in macOS 10.12.3?
1) If FileVault is encrypting your disk you will see corestoraged as a blocking process.

2) If Time Machine is encrypting an external drive with backup corestoraged will be also blocking sleep (even if the backup disk is unmounted and not connected to the Mac!!!).

➜  ~ pmset -g
System-wide power settings:
Currently in use:
standbydelay         10800
standby              1
womp                 1
halfdim              1
hibernatefile        /var/vm/sleepimage
powernap             1
gpuswitch            2
networkoversleep     0
disksleep            10
sleep                10 (sleep prevented by corestoraged)
autopoweroffdelay    28800
hibernatemode        3
autopoweroff         1
ttyskeepawake        1
displaysleep         10
acwake               0
lidwake              1


In my case, it was the Time Machine. After it finished encryption of the backup disk (took a week or so) corestoraged is not longer causing a sleep issues.