Revert "Avoid allocation storing last active time"

[jech]

This reverts commit e1c2d85.

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (v2@da3175f). Click here to learn what that means.

Additional details and impacted files

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

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

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