GH-47393: [C++][Acero] Support for multi threaded input: SortedMergeNode Version 2

[gitmodimo]

Rationale for this change

SortedMerge uses a dedicated worker thread and assumes input batches arrive in logical order. This prevents safe threaded execution and leaves backpressure, completion, stopping, and error propagation split across separate execution paths. The earlier implementation in #47394 added per-input sequencing and backpressure while retaining the existing worker thread; this PR is a new version that addresses #47393 with a single executor-managed implementation.

What changes are included in this PR?

• Replace the worker thread, process queue, and poison-pill shutdown with one executor-managed merge flow. Per-input SerialSequencingQueues restore logical ExecBatch::index order for threaded, serial, and compile-time no-thread builds.
• Add bounded upstream backpressure and downstream pause/resume handling. Completion flushes buffered rows, stopping propagates upstream, and sink-driven resume cannot race scheduler teardown.
• Select multi-row runs up to the earliest row visible on another input, allowing one selection to span multiple timestamps. Runs retain their original ExecBatch payloads until direct materialization with Arrow builders. This supports scalar payloads and bypasses Boolean slices with non-zero offsets reported in [C++][Acero] materializeColumn for boolean type overflow #48072.
• Validate ascending input order and propagate validation, materialization, and downstream errors through normal ExecPlan status handling.

Are these changes tested?

Yes. All tests pass in threaded and ARROW_ENABLE_THREADING=OFF builds. Coverage includes signed keys crossing zero, scalar payloads, deterministic sequencing under jittered delivery, and downstream backpressure, resume, and stop.

Are there any user-facing changes?

SortedMerge now supports concurrent input delivery, upstream and downstream backpressure, scalar payloads. Inputs that violate their declared ascending order return an Invalid status.
No API changes.

• GitHub Issue: [C++][Acero] Support for multi threaded input: SortedMergeNode #47393

[gitmodimo]

@ursabot please benchmark

[rok]

Benchmark runs are scheduled for commit 8470111. Watch https://buildkite.com/apache-arrow and https://conbench.arrow-dev.org for updates. A comment will be posted here when the runs are complete.

[gitmodimo]

Since codebench doesn't seem to work here are my local benchmark run on i9-14900K:

Execution	Inputs	Batch size	Baseline	Reworked	Speedup
Serial	2	1,024	269.740 ms	111.019 ms	2.43×
Serial	8	64	841.132 ms	154.442 ms	5.45×
Serial	8	1,024	785.382 ms	113.241 ms	6.94×
Serial	8	16,384	771.922 ms	108.003 ms	7.15×
Serial	32	1,024	2,848.301 ms	128.857 ms	22.10×
Threaded	2	1,024	258.336 ms	130.043 ms	1.99×
Threaded	8	64	928.467 ms	204.125 ms	4.55×
Threaded	8	1,024	802.374 ms	118.101 ms	6.79×
Threaded	8	16,384	766.673 ms	116.209 ms	6.60×
Threaded	32	1,024	2,807.121 ms	122.661 ms	22.89×