Add support for calling a url before forking for JIT warmup

[lrowe]

With deno_varnish I found it was necessary to warmup the FFI calling codepaths before the ephemeral instance fork. This might be necessary for other JITed runtimes too so I wonder if it makes sense to add this functionality into libvmod-tinykvm itself, perhaps adding settings for warmup_url and warmup_count to the json configuration.

[fwsGonzo]

Perhaps we can do that. We'd probably need to be able to design the request somewhat, but running through the codepath and not reading the response should be quite OK to add a feature for. It can be done before making forks too.

[fwsGonzo]

I've implemented a basic variant here: 927f118

In the future it should also work with the http get/set stuff that ordinarily comes from Varnish

Some thoughts:

1. The warmup phase prevents forking, which delays the first real request
2. The warmup phase shouldn't be time-consuming, ideally
3. In order to identify warmup from a real request, it should probably be specified in the struct request itself