add log messages when a hidden service consistently doesn't have any preemptive circuit ready

We basically just made up numbers for how many preemptive circuits a hidden service should have. Are popular hidden services mis-tuned?

A first step to answering this question might be to teach Tors to notice if they are too often needing to build a new circuit on the spot. They could log when it happens, and the operators could let us know they see the log and encourage us to make these parameters into config options.

Unless there's a better way to get started?