Run scheduled fires only when a URL's content changed [write]

Before each scheduled fire the platform GETs url and compares its content with the content at the last scheduled fire that ran. The same content skips the fire: it is recorded as a run with error.type precheck_unchanged and no session, and nothing is charged. New content, and the first check, run the fire. A check that cannot be completed (a redirect, a status outside 2xx, more than 256 KiB, more than 5 seconds, a host that is not on the public internet) never skips: the fire runs and the reason is kept in last_error. Manual runs, API runs and inbound-URL runs never consult it. The URL must be https on the default port, with no user name or password, and must not point at a private, loopback or link-local address (400 otherwise); it is fetched with no credentials. Setting a URL forgets what the earlier check remembered. An archived routine answers 409.