Improve iterator.Interface implementation in pgdb.queryIterator

[rumyantseva]

Description

Closes #898.

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• I ensured that the title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[vercel]

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Name	Status	Preview	Updated
ferret-db	✅ Ready (Inspect)	Visit Preview	Nov 30, 2022 at 10:36AM (UTC)

[rumyantseva]

This PR is to check a particular design/implementation idea proposed by @AlekSi, so, @AlekSi's review is required and this is why "do not merge" label is set.

[AlekSi]

I think that comment should be in the PR body before description

[codecov]

Codecov Report

Merging #1592 (675c190) into main (0a63e58) will decrease coverage by 0.10%.
The diff coverage is 91.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1592      +/-   ##
==========================================
- Coverage   68.03%   67.93%   -0.11%     
==========================================
  Files         285      285              
  Lines       13670    13655      -15     
==========================================
- Hits         9301     9277      -24     
- Misses       3435     3442       +7     
- Partials      934      936       +2     

Impacted Files	Coverage Δ
internal/handlers/pg/msg_delete.go	69.68% <ø> (-0.64%)	⬇️
internal/handlers/pg/msg_find.go	76.28% <ø> (-0.72%)	⬇️
internal/handlers/pg/pgdb/query_iterator.go	76.47% <90.90%> (-4.49%)	⬇️
internal/handlers/pg/pgdb/query.go	72.00% <100.00%> (ø)
internal/util/version/version.go	48.52% <0.00%> (-4.42%)	⬇️
internal/clientconn/conn.go	44.02% <0.00%> (-2.63%)	⬇️
internal/clientconn/listener.go	80.64% <0.00%> (-2.42%)	⬇️
internal/handlers/common/filter.go	85.43% <0.00%> (+0.58%)	⬆️

Flag	Coverage Δ
integration	63.89% <91.66%> (-0.09%)	⬇️
mongodb	14.10% <0.00%> (+<0.01%)	⬆️
pg	52.68% <91.66%> (-0.10%)	⬇️
tigris	37.51% <0.00%> (+0.01%)	⬆️
unit	26.76% <83.33%> (-0.04%)	⬇️

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[w84thesun]

Nice, it will make usage of iterator easier! Please resolve conflicts.