Revert "Avoid allocation storing last active time"

[jech]

This reverts commit e1c2d85530f0f9e96935c3598cc68b6c596c1a39.

In that commit, active time was changed from time.Time to Unix time in order to avoid allocations. Unfortunately, that has the side effect of discarding the monotonic component of time.Time, and therefore makes our code vulnerable to stepping of the system clock.

Fixes #697

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Additional details and impacted files

@@          Coverage Diff          @@
##             v2     #698   +/-   ##
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  Coverage      ?   79.55%           
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  Files         ?       42           
  Lines         ?     3581           
  Branches      ?        0           
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  Hits          ?     2849           
  Misses        ?      518           
  Partials      ?      214           

Flag	Coverage Δ
go	79.55% <100.00%> (?)
wasm	22.82% <100.00%> (?)

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