GH-40653: [C++] Avoid running more tasks in ~SerialExecutor

[pitrou]

Rationale for this change

• Add scanner tests for this

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

• GitHub Issue: [C++] Lengthy destruction of ScannerRecordBatchReader #40653

[pitrou]

@github-actions crossbow submit -g cpp

[github-actions]

⚠️ GitHub issue #40653 has been automatically assigned in GitHub to PR creator.

[github-actions]

Revision: 7e5ada6

Submitted crossbow builds: ursacomputing/crossbow @ actions-b6da829f8f

Task	Status
example-cpp-minimal-build-static
example-cpp-minimal-build-static-system-dependency
example-cpp-tutorial
test-alpine-linux-cpp
test-build-cpp-fuzz
test-conda-cpp
test-conda-cpp-valgrind
test-cuda-cpp-ubuntu-20.04-cuda-11.2.2
test-cuda-cpp-ubuntu-22.04-cuda-11.7.1
test-debian-12-cpp-amd64
test-debian-12-cpp-i386
test-fedora-39-cpp
test-ubuntu-20.04-cpp
test-ubuntu-20.04-cpp-bundled
test-ubuntu-20.04-cpp-minimal-with-formats
test-ubuntu-20.04-cpp-thread-sanitizer
test-ubuntu-22.04-cpp
test-ubuntu-22.04-cpp-20
test-ubuntu-22.04-cpp-emscripten
test-ubuntu-22.04-cpp-no-threading
test-ubuntu-24.04-cpp
test-ubuntu-24.04-cpp-gcc-13-bundled
test-ubuntu-24.04-cpp-gcc-14

[pitrou]

Hmm, it appears that one cannot simply delete the custom SerialIterator destructor, otherwise there are indeed leaks, and some tests hang.