Remove the quanta reference instant

[antifuchs]

We can always pass a 0 (which stands for an arbitrary point in time, but so does the original reference) to delta, which means there's one fewer 64 bit integer to keep around per RateLimiter.

[antifuchs]

bors merge

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 :warning:

Comparison is base (aa70cfa) 97.69% compared to head (e56736e) 97.69%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #188 +/- ## ========================================== - Coverage 97.69% 97.69% -0.01% ========================================== Files 31 31 Lines 2209 2208 -1 ========================================== - Hits 2158 2157 -1 Misses 51 51 ``` | [Impacted Files](https://app.codecov.io/gh/antifuchs/governor/pull/188?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andreas+Fuchs) | Coverage Δ | | |---|---|---| | [governor/src/clock/quanta.rs](https://app.codecov.io/gh/antifuchs/governor/pull/188?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andreas+Fuchs#diff-Z292ZXJub3Ivc3JjL2Nsb2NrL3F1YW50YS5ycw==) | `98.52% <100.00%> (-0.03%)` | :arrow_down: |

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• all_tests
• cargo_clippy
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