WL#2888: Modify sql_list.cc to remember the to-be-used memroot argument and allocate list pointers in blocks

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium

The task is to modify sql_list.cc to remember the to-be-used 
memroot argument and allocate list pointers in blocks. This 
will make the list pointers closer in memory and faster for 
the CPU to process.

Rationale
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Code cleanup task requested by Monty.