threading vs thread in solver algorithms

[ArnoStrouwen]

Some solvers use thread = OrdinaryDiffEq.True(), other use threading = true,
even though to me it looks like these keywords represent the same functionality.
It would be good to make this uniform for the next breaking release #2310 .

[ChrisRackauckas]

They should all use the Static.jl form so that it can be handled at compile time. It shouldn't be true/false either, since PolyesterThreads is an answer. IIRC there is a 3 valued logic version defined.