Warmer blocking should be enabled on cache, rather than individual warmers about cachex

[whitfin]

Perhaps required and background?

[whitfin]

I think I changed my mind here, because the naming is a little awkward. I think keeping the flag on the warmer itself does actually make sense - but i think that renaming it to something like required is much clearer.

If the warmer is required, it will be executed before the cache completes the supervision tree. This value will default to true. It makes some logic a little bit awkward in the actual initial warming, but I think this is the most obvious.