Add a background check to see if we should be specializing the worker (placeholder scenario)

[davidebbo]

Current behavior without placeholder

The scale ping causes the site to start, which causes Application_Start to run. As part of that, the host gets initialized, and everything works. Note that the http request itself never makes it to the runtime, and that doesn’t matter since the runtime is up and running.

Current behavior with placeholder

The scale ping doesn’t cause an Application_Start, since the site was started way earlier (pre-specialization). The request gets rejected, so the runtime never knows that specialization happened, since it is trigger based.

Possible fix

In addition to doing host specialization in a request triggered manner, we can have a background loop that checks every second whether we’re being specialized. We still need to keep http activation because the http trigger scenario needs to be as fast as possible. But for other scenarios like timer, an extra second is harmless.