Minor refactorings of iterators

[AlekSi]

Description

Unify how we use iterators.

Readiness checklist

• I added unit tests for new functionality or bug fixes.
• I added integration tests for new functionality or bug fixes.
• I added compatibility tests for new functionality or bug fixes.
• I made spot refactorings.
• I updated user documentation.
• 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	Dec 29, 2022 at 1:20PM (UTC)

[codecov]

Codecov Report

Merging #1718 (1cf1230) into main (af17fa7) will increase coverage by 0.05%.
The diff coverage is 85.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1718      +/-   ##
==========================================
+ Coverage   69.34%   69.39%   +0.05%     
==========================================
  Files         300      301       +1     
  Lines       14135    14124      -11     
==========================================
  Hits         9802     9802              
+ Misses       3413     3402      -11     
  Partials      920      920              

Impacted Files	Coverage Δ
internal/handlers/tigris/msg_getparameter.go	0.00% <0.00%> (ø)
internal/handlers/tigris/tigrisdb/databases.go	57.50% <0.00%> (ø)
internal/handlers/pg/msg_getparameter.go	82.47% <88.88%> (-0.69%)	⬇️
internal/handlers/pg/pgdb/query_iterator.go	76.47% <92.85%> (ø)
internal/handlers/pg/msg_delete.go	69.18% <100.00%> (-0.50%)	⬇️
internal/handlers/pg/msg_find.go	74.71% <100.00%> (+1.51%)	⬆️
internal/handlers/pg/pgdb/collections.go	64.56% <100.00%> (ø)
internal/handlers/pg/pgdb/query.go	68.68% <100.00%> (ø)
internal/handlers/pg/pgdb/transactions.go	68.51% <100.00%> (+7.15%)	⬆️
internal/types/array.go	75.00% <100.00%> (-1.28%)	⬇️
... and 7 more

Flag	Coverage Δ
integration	65.21% <71.11%> (+0.08%)	⬆️
mongodb	14.46% <0.00%> (+0.01%)	⬆️
pg	54.19% <71.11%> (+0.07%)	⬆️
tigris	39.26% <0.00%> (+0.03%)	⬆️
unit	29.12% <56.66%> (+0.08%)	⬆️

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[rumyantseva]

👍

[rumyantseva]

LGTM