Rework idle heartbeat for pull consumers #3593

Job	Run time
check (licenses)	1m 44s
check (docs)	1m 6s
check (examples)	1m 34s
check (format)	57s
check (lint)	2m 0s
check (msrv)	2m 2s
check (spelling)	6m 48s
test (ubuntu-latest, nats-main)	2m 52s
test (ubuntu-latest, nats-dev)	2m 13s
test (macos-latest, nats-main)	4m 18s
test (macos-latest, nats-dev)	6m 1s
test (windows-latest, nats-main)	7m 1s
test (windows-latest, nats-dev)	7m 56s
	46m 32s

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