Move codes for pg_backend_memory_contexts from mmgr/mcxt.c to adt/mcxtfuncs.c.
Previously the codes for pg_backend_memory_contexts were in
src/backend/utils/mmgr/mcxt.c. This commit moves them to
src/backend/utils/adt/mcxtfuncs.c so that mcxt.c basically includes
only the low-level interface for memory contexts.
Author: Atsushi Torikoshi
Reviewed-by: Michael Paquier, Fujii Masao
Discussion: https://postgr.es/m/
20200819135545.GC19121@paquier.xyz