Rework idle heartbeat for pull consumers #3591

Job	Run time
check (format)	14s
check (licenses)	1m 28s
check (docs)	1m 4s
check (examples)	1m 51s
check (lint)	1m 49s
check (msrv)	1m 54s
check (spelling)	7m 5s
test (ubuntu-latest, nats-main)	13m 50s
test (ubuntu-latest, nats-dev)	13m 27s
test (macos-latest, nats-main)	13m 51s
test (macos-latest, nats-dev)	12m 39s
test (windows-latest, nats-main)	18m 44s
test (windows-latest, nats-dev)	21m 39s
	1h 49m 35s

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