Avoid sending Link Request if the router device is powered down for duration greater than MAX_NEIGHBOR_AGE about openthread HOT 5 OPEN

If device reboots (ot::Instance re-initialized) we don't have any way to tell how long it is been off .

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@abtink we could persist the last time the device was in router role and use that to calculate the duration when thread starts again right?

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No it wont be possible since there is no notion of a (universal) clock time in OT that would persist over reboots and.or ot::Instance re-inits.

The otPlatAlarmMilliGetNow() (which maps to TimerMilli::GetNow()) is always used relative to another time value captured within the same run.

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Thanks @abtink . Is there a reason we cannot use gettimeofday for this?

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OpenThread is designed to be portable and run across different platforms and operating systems. We define a set of otPlat APIs that OpenThread uses and expects the platform to provide. To maintain this portability, we restrict the use of external library functions. gettimeofday is specific to certain platforms (POSIX/Unix). Also I think gettimeofday may not necessarily be correct upon a device reboot on an embedded linux platform.

Regarding the enhancement you are suggesting to not send initial link requests in certain situations, I am not sure it worth all the complexity it can potentially add. And it may require changes to Thread spec. The process is specified in Section "4.7.1.3. Router Synchronization after Reset".

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