Rename vacuum_cost_naptime to vacuum_cost_delay, with agreement from Jan.

doc/src/sgml/runtime.sgml

src/backend/commands/vacuum.c

src/backend/po/af.po

src/backend/po/sv.po

src/backend/po/zh_TW.po

src/backend/utils/init/globals.c

src/backend/utils/misc/guc.c

src/backend/utils/misc/postgresql.conf.sample

src/include/miscadmin.h