pgsql: Create a new 'MultiExecProcNode' call API for plan nodes that

Log Message:
-----------
Create a new 'MultiExecProcNode' call API for plan nodes that don't
return just a single tuple at a time. Currently the only such node
type is Hash, but I expect we will soon have indexscans that can return
tuple bitmaps. A side benefit is that EXPLAIN ANALYZE now shows the
correct tuple count for a Hash node.

Modified Files:
--------------
pgsql/src/backend/commands:
explain.c (r1.131 -> r1.132)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c.diff?r1=1.131&r2=1.132)
pgsql/src/backend/executor:
execProcnode.c (r1.48 -> r1.49)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execProcnode.c.diff?r1=1.48&r2=1.49)
instrument.c (r1.11 -> r1.12)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/instrument.c.diff?r1=1.11&r2=1.12)
nodeHash.c (r1.92 -> r1.93)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeHash.c.diff?r1=1.92&r2=1.93)
nodeHashjoin.c (r1.70 -> r1.71)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeHashjoin.c.diff?r1=1.70&r2=1.71)
pgsql/src/include/executor:
executor.h (r1.117 -> r1.118)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/executor.h.diff?r1=1.117&r2=1.118)
hashjoin.h (r1.35 -> r1.36)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/hashjoin.h.diff?r1=1.35&r2=1.36)
instrument.h (r1.10 -> r1.11)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/instrument.h.diff?r1=1.10&r2=1.11)
nodeHash.h (r1.36 -> r1.37)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/nodeHash.h.diff?r1=1.36&r2=1.37)