MINOR: acme: opportunistic DNS check for dns-persist-01 to skip challenge-ready steps

MINOR: acme: opportunistic DNS check for dns-persist-01 to skip challenge-ready steps

For dns-persist-01, the "_validation-persist.<domain>" TXT record is set once
and never changes between renewals. Add an initial opportunistic DNS check
(ACME_INITIAL_RSLV_TRIGGER / ACME_INITIAL_RSLV_READY states) that runs before
the challenge-ready conditions are evaluated. If all domains already have the
TXT record, the challenge is submitted immediately without going through the
cli/delay/dns challenge-ready steps, making renewals faster once the record is
in place.

The new ACME_RDY_INITIAL_DNS flag is automatically set for
dns-persist-01 in cond_ready.