Add RESET CONNECTION, to reset all aspects of a session.

doc/src/sgml/ref/reset.sgml

src/backend/catalog/namespace.c

src/backend/commands/async.c

src/backend/commands/prepare.c

src/backend/parser/gram.y

src/backend/utils/misc/guc.c

src/backend/utils/mmgr/portalmem.c

src/include/catalog/namespace.h

src/include/commands/async.h

src/include/commands/prepare.h

src/include/utils/portal.h

src/interfaces/ecpg/preproc/preproc.y