drainRelayOutboxes drained one gateway's outbox and delivered its envelopes
before draining the next gateway, and delivered every envelope one after
another. One long turn — bot_relay.deliver may take up to
RELAY_DELIVER_TIMEOUT_MS, 25 minutes — therefore held every other bot's mail:
a sibling gateway's envelope sat unclaimed in its outbox, and since the
gateway checks the envelope's age against bot_mode.envelope_ttl_seconds
(15 minutes) at the claim, the sender's waiter received queued_expired for a
message nothing was wrong with; envelopes that did get claimed still waited
their turn behind unrelated deliveries, against a finite waiter.
Claim every gateway's outbox first, then deliver in lanes keyed by target
connection and profile: a lane runs its envelopes in order, one turn at a
time (the target gateway serialises that profile's turns behind its turn
lock anyway), and lanes run concurrently.
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