pgsql: Improve hash join to discard input tuples immediately if they

Log Message:
-----------
Improve hash join to discard input tuples immediately if they can't
match because they contain a null join key (and the join operator is
known strict). Improves performance significantly when the inner
relation contains a lot of nulls, as per bug #2930.

Modified Files:
--------------
pgsql/src/backend/executor:
nodeHash.c (r1.108 -> r1.109)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeHash.c.diff?r1=1.108&r2=1.109)
nodeHashjoin.c (r1.86 -> r1.87)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeHashjoin.c.diff?r1=1.86&r2=1.87)
pgsql/src/include/executor:
hashjoin.h (r1.42 -> r1.43)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/hashjoin.h.diff?r1=1.42&r2=1.43)
nodeHash.h (r1.42 -> r1.43)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/nodeHash.h.diff?r1=1.42&r2=1.43)