issues/296: Fix bug where acme certificates were not cached in memory

[komuw]

• Before(due to a feature misuse bug), certificates would be fetched from disk for each request.

• Fixes: https://github.com/komuw/ong/issues/296

[codecov-commenter]

Codecov Report

Patch coverage: 85.71% and project coverage change: +0.08 :tada:

Comparison is base (e4c4d8f) 73.64% compared to head (3f848c6) 73.73%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #304 +/- ## ========================================== + Coverage 73.64% 73.73% +0.08% ========================================== Files 42 42 Lines 4799 4804 +5 ========================================== + Hits 3534 3542 +8 + Misses 1015 1011 -4 - Partials 250 251 +1 ``` | [Impacted Files](https://app.codecov.io/gh/komuw/ong/pull/304?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Komu+Wairagu) | Coverage Δ | | |---|---|---| | [internal/acme/acme.go](https://app.codecov.io/gh/komuw/ong/pull/304?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Komu+Wairagu#diff-aW50ZXJuYWwvYWNtZS9hY21lLmdv) | `62.41% <ø> (-0.69%)` | :arrow_down: | | [server/tls\_conf.go](https://app.codecov.io/gh/komuw/ong/pull/304?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Komu+Wairagu#diff-c2VydmVyL3Rsc19jb25mLmdv) | `57.53% <85.71%> (+10.47%)` | :arrow_up: |

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