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GH-36092: [C++] Simplify concurrency in as-of-join node (#36094)

### What changes are included in this PR?

The key hasher invalidation and memo-store time updating are moved to the processing thread.

### Are these changes tested?

Yes, by existing tests.

### Are there any user-facing changes?

No.
* Closes: #36092

Authored-by: Yaron Gvili <rtpsw@hotmail.com>
Signed-off-by: Li Jin <ice.xelloss@gmail.com>