Auto-tune effective_cache size to be 4x shared buffers

doc/src/sgml/config.sgml

src/backend/optimizer/path/costsize.c

src/backend/postmaster/postmaster.c

src/backend/utils/misc/guc.c

src/include/optimizer/cost.h

src/include/utils/guc.h

src/test/regress/expected/join.out

src/test/regress/sql/join.sql