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)