pgsql: Add buffer access counters to pg_stat_statements.

Log Message:
-----------
Add buffer access counters to pg_stat_statements.

This uses the same infrastructure with EXPLAIN BUFFERS to support
{shared|local}_blks_{hit|read|written} andtemp_blks_{read|written}
columns in the pg_stat_statements view. The dumped file format
also updated.

Thanks to Robert Haas for the review.

Modified Files:
--------------
    pgsql/contrib/pg_stat_statements:
        pg_stat_statements.c (r1.11 -> r1.12)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_stat_statements/pg_stat_statements.c?r1=1.11&r2=1.12)
        pg_stat_statements.sql.in (r1.1 -> r1.2)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_stat_statements/pg_stat_statements.sql.in?r1=1.1&r2=1.2)
    pgsql/doc/src/sgml:
        pgstatstatements.sgml (r1.5 -> r1.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/pgstatstatements.sgml?r1=1.5&r2=1.6)