Derive both the flaky-server drop count and the zero-delay schedule
from RETRY_DELAYS_SECS.len() so the tests keep exercising their
intended invariants — "succeeds on final attempt" and "gives up after
all attempts fail" — if the production retry schedule ever changes.
Also: rename fail_first → drop_first_n to match drop(sock); swap the
giveup test's empty body for an "unreachable" sentinel so a regression
that accidentally served couldn't silently match Some("").
On cold start, reqwest's getaddrinfo can race numa's own first-query
cold-path latency — resolver timeout fires before numa warms its
upstream DoH connection. Wrap each blocklist fetch in 3 retries with
2s/10s/30s backoff; by the second attempt, the upstream is warm and
subsequent getaddrinfos succeed in <100ms.
Also: parallelize fetches across lists via join_all (different hosts,
no warming dependency), walk the full error source chain so reqwest
failures surface the underlying cause, and parameterize retry delays
for unit-test speed.
