Add GUC log_lock_waits to log long wait times.

doc/src/sgml/config.sgml

src/backend/storage/lmgr/deadlock.c

src/backend/storage/lmgr/proc.c

src/backend/utils/misc/guc.c

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

src/include/storage/lock.h

src/include/storage/proc.h