Use the new "Slab" context for some allocations in reorderbuffer.h.

Use the new "Slab" context for some allocations in reorderbuffer.h.

Note that this change alone does not yet fully address the performance
problems triggering this work, a large portion of the slowdown is
triggered by the tuple allocator, which isn't converted to the new
allocator.  It would be possible to do so, but using evenly sized
objects, like both the current implementation in reorderbuffer.c and
slab.c, wastes a fair amount of memory.  A later patch by Tomas will
introduce a better approach.

Author: Tomas Vondra
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/d15dff83-0b37-28ed-0809-95a5cc7292ad@2ndquadrant.com