Add NOWAIT option to LOCK command

doc/src/sgml/ref/lock.sgml

src/backend/access/heap/heapam.c

src/backend/commands/lockcmds.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/gram.y

src/backend/parser/keywords.c

src/include/access/heapam.h

src/include/nodes/parsenodes.h