[PATCH] VCL temperature improvements

[Dridi Boukelmoune]

Hi,

This is hopefully the closure of work I started during VDD15Q3, with a
design refined during VDD15Q4. Some of the post-Q3 patches have
already been pushed to the master branch, and the remaining have been
split and should be easier to review.

If a release manager happens to look at this mail, here is a list of
commits that would need to be back-ported to 4.1 (I can do that if
asked) along with the attached patches:

* 513aee7  Add a reminder for a VRT_MINOR_VERSION bump
* ae5a342  Polish
* 6a6358f  Polish
* 89ce06a  Wording
* 502a871  Coverage for the cooling state
* 8bf2b0a  Hold a VCL reference in the debug vmod
* 28eec51  Make event handling more readable in vmod-debug
* 57a5b9c  Introduce a new "busy" VCL temperature
* 2b6bf4f  Start documenting that VCL cool down should go fast
* 3162873  Don't create backends on cooling VCLs, return NULL
* 50aeefc  Allow VMODs to hold a reference on a warm VCL
* 16b8375  Clarify VRT_delete_backend a bit
* c503bb7  Avoid connection attempts to ${bad_ip}:80
* efd5dfd  VMODs handling of VCL_EVENT_COLD must be failsafe
* fbd1c18  Document that VCL_EVENT_USE is deprecated
* 445854c  Reserve space for the "cooling" state in vcl.list

I'm not sure when I can join on IRC to discuss those patches, but I
tried to convey as much information as I could in the commit messages.
If the patches are merged, let me know and I'll write changelog
entries.

Best,
Dridi
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