Make it harder to misuse iterators

[AlekSi]

Description

Just close them.

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR 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.

[codecov]

Codecov Report

Merging #2428 (0a821db) into main (53a76ec) will decrease coverage by 37.47%.
The diff coverage is 0.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2428       +/-   ##
===========================================
- Coverage   64.34%   26.88%   -37.47%     
===========================================
  Files         392      393        +1     
  Lines       19251    19289       +38     
===========================================
- Hits        12388     5185     -7203     
- Misses       5961    13549     +7588     
+ Partials      902      555      -347     

Impacted Files	Coverage Δ
internal/handlers/common/filter_iterator.go	0.00% <0.00%> (-100.00%)	⬇️
internal/handlers/common/limit_iterator.go	0.00% <0.00%> (-84.00%)	⬇️
internal/handlers/common/projection_iterator.go	0.00% <0.00%> (-75.00%)	⬇️
internal/handlers/common/skip_iterator.go	0.00% <0.00%> (-88.00%)	⬇️
internal/handlers/common/sort_iterator.go	0.00% <0.00%> (-100.00%)	⬇️
internal/handlers/pg/msg_find.go	0.00% <0.00%> (-70.41%)	⬇️
internal/handlers/tigris/msg_find.go	0.00% <0.00%> (ø)
internal/util/iterator/closer.go	0.00% <0.00%> (ø)
internal/util/iterator/slice.go	100.00% <ø> (ø)
internal/util/iterator/values.go	94.73% <ø> (-2.64%)	⬇️
... and 1 more

... and 118 files with indirect coverage changes

Flag	Coverage Δ
integration	4.71% <0.00%> (-52.62%)	⬇️
mongodb	4.71% <0.00%> (-0.01%)	⬇️
pg	?
unit	25.54% <0.00%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[w84thesun]

I think one could create Closer and still forget to call closer.Close but that change still would simplify the use of iterators. 😄

[AlekSi]

I think one could create Closer and still forget to call closer.Close

That would trigger a panic inside finalizer. I will add it to Closer too to make it easier to spot