Shared/Database: Improve dead-lock error handling

Improve dead-lock error handling by making sure only 1 thread at time retries to execute a transaction so they don't keep dead-locking each other.

(cherry picked from commit 62001360dd)

src/server/shared/Database/Transaction.cpp

@@ -19,6 +19,8 @@
 #include "Transaction.h"
 #include <mysqld_error.h>
 
+std::mutex TransactionTask::_deadlockLock;
+
 //- Append a raw ad-hoc query to the transaction
 void Transaction::Append(const char* sql)
 {
@@ -81,6 +83,8 @@ bool TransactionTask::Execute()
 
     if (errorCode == ER_LOCK_DEADLOCK)
     {
+        // Make sure only 1 async thread retries a transaction so they don't keep dead-locking each other
+        std::lock_guard<std::mutex> lock(_deadlockLock);
         uint8 loopBreaker = 5;  // Handle MySQL Errno 1213 without extending deadlock to the core itself
         for (uint8 i = 0; i < loopBreaker; ++i)
             if (!m_conn->ExecuteTransaction(m_trans))