Enabling concurrent spilling

[nirandaperera]

[C++] Allow concurrent SpillManager::spill() invocations

SpillManager serialized all spill() calls behind a single std::mutex, even though spill work is long-running (D2H copies, I/O) and only the rarely-touched function registry actually needed mutual exclusion. This switches the lock to a std::shared_mutex so spill() takes a shared lock and runs in parallel, while add_spill_function / remove_spill_function still take an exclusive lock — which preserves the drain-on-remove guarantee that callers like Shuffler rely on for safe teardown.

• spill() now takes a shared lock; concurrent callers no longer serialize on the registry mutex.
• add_spill_function / remove_spill_function take an exclusive lock, so in-flight spillers are drained before either returns.
• Added SpillManager.ConcurrentSpill test that uses a rendezvous to prove spill() calls actually overlap (it would fail on the old exclusive mutex).
• Audited the three registered C++ spill functions (Shuffler::spill, AllGather::spill, spill_messages); all already lock their own state for concurrent self-invocation, so no caller-side changes are required.