AP_ESC_Telem: fix other timeout race #29028
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The RPM telemetry data structure can get updated by another thread at any time. This can cause (now - last_update_us) to be negative, which looks like the data is nearly UINT32_MAX microseconds stale. To fix this, we copy the last_update_us value before we get the current time so it's guaranteed to be positive. We also minimize the number of places we explicitly check the timestamp and rely on the `data_valid` where possible to minimize the performance impact of this change.
The timeout for non-RPM telemetry is vulnerable to a similar race as the RPM. This change makes the timeout logic consistent between the two.
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Prerequisite PR: #28999
The timeout for non-RPM telemetry is vulnerable to a similar race to the RPM one above. This PR makes the timeout logic consistent between the two, adding a validity flag to track the timeout instead of explicitly subtracting timestamps (one of which is volatile).
I broke this PR into a separate dependent PR in case this is considered too risky of a change. I have an alternative PR ready, mirroring how Pete is binding AP_Servo_Telem in the soon-to-be-merged #28857. That PR would be guaranteed not to change any behavior outside of Lua, however, I think this PR is vastly a better approach.