Attach to the DSA only when we actually access the DSA

[jeltz]

Clean up some confusion about "attaching" to shared memory by changing the function names and making sure to only call pgsm_attach_dsa() before accessing the query DSA.

PG-0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.22%. Comparing base (643897b) to head (b4655c4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #810      +/-   ##
==========================================
+ Coverage   88.07%   88.22%   +0.14%     
==========================================
  Files           3        3              
  Lines        1300     1299       -1     
  Branches      180      180              
==========================================
+ Hits         1145     1146       +1     
+ Misses         76       75       -1     
+ Partials       79       78       -1     

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