For (real) backends, we used to assign the probe after adding the
director via VRT_AddDirector(). For an already warm vcl, this lead to
a VCL_WARM event missed because the event was issued by
VRT_AddDirector() when the probe did not exist yet.

We fix this by preparing all of the backend before adding the
director, undoing the work for a failure doing do.

In addition, if adding a backend while the vcl is cold, we need to set
the initial director state based on the probe configuration after
adding the director.

We avoid the overhead of the vcl temperature lock because no harm is
done: If, after the director was added, the temperature goes cold, we
just run an additional update, and if it goes warm, the update will
already have happened.

Likely related to https://github.com/nigoroll/libvmod-dynamic/issues/35