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

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
---------
Code cleanup task requested by Monty.