Simplify issuer storage

[phbnf]

88

• Inline calls to Exists and simplify the API
• Write directly doing a read first: we're talking about saving cents here for 15k entries, it's not worth the complexity
• crack open the precondition errors to make sure it's really failing because of the precondition we pass

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 31 lines in your changes missing coverage. Please review.

Project coverage is 34.52%. Comparing base (46ec9c2) to head (cb8a9e6). Report is 84 commits behind head on main.

Files	Patch %	Lines
personalities/sctfe/storage.go	0.00%	25 Missing :warning:
personalities/sctfe/storage/gcp/issuers.go	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #173 +/- ## ========================================== - Coverage 35.80% 34.52% -1.28% ========================================== Files 16 34 +18 Lines 1363 2966 +1603 ========================================== + Hits 488 1024 +536 - Misses 801 1831 +1030 - Partials 74 111 +37 ```

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