Revert "Avoid allocation storing last active time"

[jech]

This reverts commit edb69295c04429931ede6a3df01d49520a1b31dc.

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.

This carries over #698 to v3. Fixes #697.

[codecov[bot]]

Codecov Report

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

Project coverage is 79.19%. Comparing base (899594c) to head (4636318).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #699   +/-   ##
=======================================
  Coverage   79.19%   79.19%           
=======================================
  Files          41       41           
  Lines        3739     3739           
=======================================
  Hits         2961     2961           
  Misses        547      547           
  Partials      231      231           

Flag	Coverage Δ
go	79.19% <100.00%> (ø)
wasm	22.13% <100.00%> (ø)

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