MEDIUM: acme: split the initial delay from the retry DNS delay

MEDIUM: acme: split the initial delay from the retry DNS delay

The previous ACME_RSLV_WAIT state served a dual role: it applied the
initial dns-delay before the first DNS probe and also handled the
delay between retries. There was no way to simply wait a fixed delay
before submitting the challenge without also triggering DNS pre-checks.

Replace ACME_RSLV_WAIT with two distinct states:
  - ACME_INITIAL_DELAY: an optional initial wait before proceeding,
    only applied when "challenge-ready" includes the new "delay" keyword
  - ACME_RSLV_RETRY_DELAY: the delay between resolution retries, always
    applied when DNS pre-checks are in progress

The new "delay" keyword in "challenge-ready" can be used standalone
(wait then submit the challenge directly) or combined with "dns" (wait
then start the DNS pre-checks). When "delay" is not set, the first DNS
probe fires immediately.

Update the documentation accordingly.