WL#6037: Decrease malloc use in : alloc_root

Affects: Server-Prototype Only   —   Status: Un-Assigned   —   Priority: Medium

my_malloc calls made when doing a normal PK lookup SELECT query

From Mikaels notes:

alloc_root
I think two memroots are allocated one per query on THD object +
1 per table used in query. Both get at least one malloc call since
the first buffer isn't cached in THD object or TABLE object