Merge bitcoin#23397: Avoid excessive lock contention in CCheckQueue::Add

459e208 Exit early for an empty vChecks in CCheckQueue::Add (Hennadii Stepanov)
c43aa62 Avoid excessive lock contention in CCheckQueue::Add (Hennadii Stepanov)

Pull request description:

  This PR significantly reduces lock contention in the `CCheckQueue` class by releasing a mutex before calling `std::condition_variable::notify_one` and `std::condition_variable::notify_all`.

  From C++ [docs](https://en.cppreference.com/w/cpp/thread/condition_variable/notify_one):
  > The notifying thread does not need to hold the lock on the same mutex as the one held by the waiting thread(s); in fact doing so is a pessimization, since the notified thread would immediately block again, waiting for the notifying thread to release the lock.

  Related to:
  - bitcoin#23167
  - bitcoin#23223

ACKs for top commit:
  martinus:
    ACK 459e208, codereview and tested. I first thought this introduced a segfault in `psbt_wallet_tests/psbt_updater_test` because that test failed for me, but thats a different issue fixed in bitcoin#23403.
  vasild:
    ACK 459e208
  theStack:
    Code-review ACK 459e208

Tree-SHA512: c197858656392ba3ebcd638d713cf93c9fb48b7b3bad193209490d2828f9c7e3ae4dee6f84674f2f34dceed894139562e29579ee7299e06756c8c990caddc5ed

src/checkqueue.h

@@ -165,16 +165,24 @@ class CCheckQueue
     //! Add a batch of checks to the queue
     void Add(std::vector<T>& vChecks)
     {
-        LOCK(m_mutex);
-        for (T& check : vChecks) {
-            queue.push_back(T());
-            check.swap(queue.back());
+        if (vChecks.empty()) {
+            return;
         }
-        nTodo += vChecks.size();
-        if (vChecks.size() == 1)
+
+        {
+            LOCK(m_mutex);
+            for (T& check : vChecks) {
+                queue.emplace_back();
+                check.swap(queue.back());
+            }
+            nTodo += vChecks.size();
+        }
+
+        if (vChecks.size() == 1) {
             m_worker_cv.notify_one();
-        else if (vChecks.size() > 1)
+        } else {
             m_worker_cv.notify_all();
+        }
     }
 
     //! Stop all of the worker threads.