pgsql: Augment EXPLAIN output with more details on Hash nodes.

Log Message:
-----------
Augment EXPLAIN output with more details on Hash nodes.

We show the number of buckets, the number of batches (and also the original
number if it has changed), and the peak space used by the hash table.  Minor
executor changes to track peak space used.

Modified Files:
--------------
    pgsql/src/backend/commands:
        explain.c (r1.199 -> r1.200)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c?r1=1.199&r2=1.200)
    pgsql/src/backend/executor:
        nodeHash.c (r1.126 -> r1.127)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeHash.c?r1=1.126&r2=1.127)
    pgsql/src/include/executor:
        hashjoin.h (r1.52 -> r1.53)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/hashjoin.h?r1=1.52&r2=1.53)