Cost based vacuum delay feature.

src/backend/access/nbtree/nbtree.c

src/backend/commands/vacuumlazy.c

src/backend/storage/buffer/bufmgr.c

src/backend/storage/buffer/freelist.c

src/backend/tcop/postgres.c

src/backend/utils/init/globals.c

src/backend/utils/misc/guc.c

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

src/include/miscadmin.h