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GH-44846: [C++] Fix thread-unsafe access in ConcurrentQueue::UnsyncFront (#44849)

### Rationale for this change

The `UnsyncFront` method claims that it can access a `std::deque`'s front element without synchronizing with another thread that would call `std::deque::push_back`, but `std::deque::front` can actually be implemented in terms of `std::deque::begin` while `std::deque::push_back` is specified to invalidate iterators.

In the end, `UnsyncFront` is concretely thread-unsafe even though it might ideally be thread-safe. This shows up as occasional Thread Sanitizer failures.

### What changes are included in this PR?

Replace `UnsyncFront` with a thread-safe `Front` method.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

* GitHub Issue: #44846

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>