Demote ensure_computing to function

[fjetter]

Follow up to #5128

[GPUtester]

Can one of the admins verify this patch?

[mrocklin]

Oh wow. I didn't realize that we were no longer doing anything async in this method. +1

[fjetter]

I didn't realize that we were no longer doing anything async in this method

the execute is scheduled as part of the transition and the deserialization stuff is part of the execute itself. that let's us deal with possible deserialization exceptions the same way as for an ordinary compute failure (regarding the state machine). that's the essence of #5128

FWIW I would like to get rid of this function entirely since I believe everything that needs to be done here can be done as part of a transition which should make the entire thing a bit more streamlined imo. Currently, it's still required for pausing so we'll need to keep it regardless of my preference :)