Add HTTP idling config values.
Set idle timer for closing idle HTTP drains.
Turns out ready_to_send isn't an exported state.
Check that idle drains are closed in http drain test.
Replace cancel_reconnect with more general cancel_timeout.
Track idle_tref in State record of HTTP drain module.
Use timer:now_diff instead of custom now_us function.
Return modified State from close_if_idle.

Also multiply ms instead of dividing us in order to avoid floats.

Same as 15366ff, but for HTTP drains.
Match TRef on IDLE_TIMEOUT_MSG with the idle_tref in State record.
Merge branch 'close-idle-http-drains'

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