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Backend idle connections are purged on a recurring occurence during the process lifetime. An estimated number of needed connections is calculated and the excess is removed periodically. Before this patch, purge was done directly using the idle then the safe connection tree of a server instance. This has a major drawback to take no account of a specific ordre and it may removed functional connections while leaving ones which will fail on the next reuse. The problem can be worse when using criteria to differentiate idle connections such as the SSL SNI. In this case, purge may remove connections with a high rate of reusing while leaving connections with criteria never matched once, thus reducing drastically the reuse rate. To improve this, introduce an alternative storage for idle connection used in parallel of the idle/safe trees. Now, each connection inserted in one of this tree is also inserted in the new list at `srv_per_thread.idle_conn_list`. This guarantees that recently used connection is present at the end of the list. During the purge, use this list instead of idle/safe trees. Remove first connection in front of the list which were not reused recently. This will ensure that connection that are frequently reused are not purged and should increase the reuse rate, particularily if distinct idle connection criterias are in used.