Fix deadlock in conversations

[grisha2217]

This happens when user trying send two or more messages in short time (like 2 messages in 1 second)
XenForo 1.5.24
Reward: 50$

Zend_Db_Statement_Mysqli_Exception: Mysqli statement execute error : Deadlock found when trying to get lock; try restarting transaction - library/Zend/Db/Statement/Mysqli.php:214
Generated By: user, 2 minutes ago

#0 library/Zend/Db/Statement.php(297): Zend_Db_Statement_Mysqli->_execute(Array)
#1 library/Zend/Db/Adapter/Abstract.php(479)...
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