WL#6034: Decrease malloc use in : init_sql_alloc

my_malloc calls made when doing a normal PK lookup SELECT query.

init_sql_alloc

Called from 
open_tables.open_and_lock_tables.execute_sqlcom_select.mysql_execute_command

Potential Remedy:
Cache first 8k buffer in memroot on THD object between queries

This issue has been handled as part of work on bug #13116518 - "OPEN_TABLES()
SHOULD NOT ALLOCATE AND FREE NEW_FRM_MEM WITHOUT NEEDING TO".