Power feature design

[MrKevinWeiss]

So after a bit of talking we determined the following:

• Power should be an opt-in feature as developers would probably not need or use it.
• Better if we don't need caching
• Every command would start by turning everything on (or maybe turn on as much as possible before error)
• After the acquire of all devices in a command, all unused boards would turn off
• After the release of all devices in a command, all unused boards would turn off
• Once all boards in that batch are done, other remaining devices could attempt to be processed

Assumptions:

• Boards will not be changed when running a command
• The time after turning on all boards and acquiring would block any other commands from being used

More eval of uhubctrl will be needed to see if groups of devices can be supported.