Optimize Date plugin again (refresh timezone only once a minute)

This makes the Date plugin approximately twice as fast, and makes xmobar
up to about 5–10 % faster if Date is the only active plugin. (If more
expensive plugins like Network or MultiCpu are used, it doesn't make any
measurable difference.)

Micro-benchmark results on my HW:

    Date Benchmarks/Date                     mean 2.833 μs  ( +- 16.08 ns  )
    Date Benchmarks/DateZonedTime            mean 5.020 μs  ( +- 32.91 ns  )
    Date Benchmarks/DateWithTimeZone         mean 2.827 μs  ( +- 20.52 ns  )

(DateZonedTime is the original implementation and DateWithTimeZone is
the implementation we had since 0.34 which never refreshes timezone.)

Real-life measurements (done overnight on an idle laptop, with all
measured xmobars running in parallel to ensure comparable conditions;
xmobars configured to only display date and with rate 10 — once per
second):

    $ time timeout 6h xmobar .xmobarrc-DateZonedTime
    real    360m0,010s
    user    0m9,867s
    sys     0m4,644s

    (9.867 + 4.644) / (360 * 60) = 0.000672

    $ time timeout 6h xmobar .xmobarrc-Date
    real    360m0,008s
    user    0m9,535s
    sys     0m4,327s

    (9.535 + 4.327) / (360 * 60) = 0.000642

    $ time timeout 6h xmobar .xmobarrc-Date-10m
    real    360m0,010s
    user    0m9,780s
    sys     0m4,215s

    (9.780 + 4.215) / (360 * 60) = 0.000648

    $ time timeout 6h xmobar .xmobarrc-DateWithTimeZone
    real    360m0,006s
    user    0m9,658s
    sys     0m4,166s

    (9.658 + 4.166) / (360 * 60) = 0.000640

(.xmobarrc-Date-10m is the proposed implementation, but with timezone
refresh every 10 minutes instead of every 1 minute)

Interpretation of these results:

 * refreshing xmobar with just date takes around 650 μs
 * that is xmobar with just date uses around 0.065 % of CPU time
 * refreshing timezone takes additional cca 30 μs

When we only refresh timezone once a minute, these 30 μs become 0.5 μs
amortized, and that should be acceptable to even the most dedicated
perfectionist :-)

Fixes: a58e32f7c8af ("Revert "Optimize date plugin"")
Fixes: 878db3908060 ("Optimize date plugin")
Co-authored-by: Sibi Prabakaran <sibi@psibi.in>

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