Allow to change (reduce) max queue size for common elastic queue

Add same method as in TFastElasticQueue, and test.
commit_hash:0a2b618325e57c32fd269254a7dbe912849c3f10

Author: kulikov

library/cpp/threading/equeue/equeue.cpp

@@ -10,7 +10,7 @@ size_t TElasticQueue::ObjectCount() const {
 }
 
 bool TElasticQueue::TryIncCounter() {
-    if (++GuardCount_ > MaxQueueSize_) {
+    if (++GuardCount_ > CurrentMaxQueueSize_) {
         --GuardCount_;
         return false;
     }
@@ -68,6 +68,7 @@ bool TElasticQueue::Add(IObjectInQueue* obj) {
 
 void TElasticQueue::Start(size_t threadCount, size_t maxQueueSize) {
     MaxQueueSize_ = maxQueueSize;
+    CurrentMaxQueueSize_ = maxQueueSize;
     SlaveQueue_->Start(threadCount, maxQueueSize);
 }
 

library/cpp/threading/equeue/equeue.h

@@ -17,13 +17,20 @@ class TElasticQueue: public IThreadPool {
     void Stop() noexcept override;
 
     size_t ObjectCount() const;
+
+    void SetCurrentMaxQueueSize(size_t v) {
+        Y_ENSURE(v <= MaxQueueSize_);
+        CurrentMaxQueueSize_ = v;
+    }
 private:
     class TDecrementingWrapper;
 
     bool TryIncCounter();
 private:
     THolder<IThreadPool> SlaveQueue_;
+
     size_t MaxQueueSize_ = 0;
+    std::atomic<size_t> CurrentMaxQueueSize_ = 0;
     std::atomic<size_t> ObjectCount_ = 0;
     std::atomic<size_t> GuardCount_ = 0;
 };