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 ⚠️
personalities/sctfe/storage/gcp/issuers.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            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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[AlCutter]

Just a new nits mostly, and missing locking.