GH-36482: [C++][CI] Fix sporadic test failures in AsofJoinBasicTest (#36499)

### What changes are included in this PR?

The key hasher is invalidated before the first invocation of `GetKey` (via `GetLatestKey`) after a new batch arrives. In the pre-PR code, this invalidation happens within `Advance`, which is called from `AdvanceAndMemoize` only after `GetLatestKey` is called. The change adds synchronization between the input-receiving- and processing- threads, because avoiding that would require a more complicated and brittle change, e.g., one that involves detecting in the processing thread when a new batch was added to the queue in order to invalidate the key hasher at that time.

### Are these changes tested?

Yes, by existing tests.

### Are there any user-facing changes?

No.

**This PR contains a "Critical Fix".**
* Closes: #36482

Authored-by: Yaron Gvili <[email protected]>
Signed-off-by: Weston Pace <[email protected]>

Author: rtpsw

cpp/src/arrow/acero/asof_join_node.cc

@@ -524,7 +524,7 @@ class KeyHasher {
   size_t index_;
   std::vector<col_index_t> indices_;
   std::vector<KeyColumnMetadata> metadata_;
-  const RecordBatch* batch_;
+  std::atomic<const RecordBatch*> batch_;
   std::vector<HashType> hashes_;
   LightContext ctx_;
   std::vector<KeyColumnArray> column_arrays_;
@@ -819,7 +819,6 @@ class InputState {
         have_active_batch &= !queue_.TryPop();
         if (have_active_batch) {
           DCHECK_GT(queue_.UnsyncFront()->num_rows(), 0);  // empty batches disallowed
-          key_hasher_->Invalidate();  // batch changed - invalidate key hasher's cache
           memo_.UpdateTime(GetTime(queue_.UnsyncFront().get(), 0));  // time changed
         }
       }
@@ -897,7 +896,8 @@ class InputState {
 
   Status Push(const std::shared_ptr<arrow::RecordBatch>& rb) {
     if (rb->num_rows() > 0) {
-      queue_.Push(rb);  // only after above updates - push batch for processing
+      key_hasher_->Invalidate();  // batch changed - invalidate key hasher's cache
+      queue_.Push(rb);            // only now push batch for processing
     } else {
       ++batches_processed_;  // don't enqueue empty batches, just record as processed
     }