pgsql: Add an EXPLAIN (BUFFERS) option to show buffer-usage statistics.

Log Message:
-----------
Add an EXPLAIN (BUFFERS) option to show buffer-usage statistics.

This patch also removes buffer-usage statistics from the track_counts
output, since this (or the global server statistics) is deemed to be a better
interface to this information.

Itagaki Takahiro, reviewed by Euler Taveira de Oliveira.

Modified Files:
--------------
    pgsql/contrib/auto_explain:
        auto_explain.c (r1.9 -> r1.10)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/auto_explain/auto_explain.c?r1=1.9&r2=1.10)
    pgsql/contrib/pg_stat_statements:
        pg_stat_statements.c (r1.7 -> r1.8)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_stat_statements/pg_stat_statements.c?r1=1.7&r2=1.8)
    pgsql/doc/src/sgml:
        auto-explain.sgml (r1.5 -> r1.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/auto-explain.sgml?r1=1.5&r2=1.6)
    pgsql/doc/src/sgml/ref:
        explain.sgml (r1.47 -> r1.48)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/explain.sgml?r1=1.47&r2=1.48)
    pgsql/src/backend/commands:
        copy.c (r1.318 -> r1.319)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/copy.c?r1=1.318&r2=1.319)
        explain.c (r1.195 -> r1.196)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c?r1=1.195&r2=1.196)
        tablecmds.c (r1.309 -> r1.310)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.309&r2=1.310)
    pgsql/src/backend/executor:
        execMain.c (r1.337 -> r1.338)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c?r1=1.337&r2=1.338)
        execProcnode.c (r1.68 -> r1.69)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execProcnode.c?r1=1.68&r2=1.69)
        functions.c (r1.137 -> r1.138)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/functions.c?r1=1.137&r2=1.138)
        instrument.c (r1.22 -> r1.23)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/instrument.c?r1=1.22&r2=1.23)
        spi.c (r1.211 -> r1.212)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c?r1=1.211&r2=1.212)
    pgsql/src/backend/storage/buffer:
        buf_init.c (r1.82 -> r1.83)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/buf_init.c?r1=1.82&r2=1.83)
        bufmgr.c (r1.252 -> r1.253)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c?r1=1.252&r2=1.253)
        localbuf.c (r1.87 -> r1.88)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/localbuf.c?r1=1.87&r2=1.88)
    pgsql/src/backend/storage/file:
        buffile.c (r1.34 -> r1.35)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/file/buffile.c?r1=1.34&r2=1.35)
    pgsql/src/backend/tcop:
        postgres.c (r1.575 -> r1.576)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c?r1=1.575&r2=1.576)
        pquery.c (r1.132 -> r1.133)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/pquery.c?r1=1.132&r2=1.133)
    pgsql/src/include/commands:
        explain.h (r1.43 -> r1.44)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/explain.h?r1=1.43&r2=1.44)
    pgsql/src/include/executor:
        execdesc.h (r1.40 -> r1.41)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/execdesc.h?r1=1.40&r2=1.41)
        executor.h (r1.164 -> r1.165)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/executor.h?r1=1.164&r2=1.165)
        instrument.h (r1.20 -> r1.21)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/instrument.h?r1=1.20&r2=1.21)
    pgsql/src/include/nodes:
        execnodes.h (r1.213 -> r1.214)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/execnodes.h?r1=1.213&r2=1.214)
    pgsql/src/include/storage:
        buf_internals.h (r1.102 -> r1.103)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/buf_internals.h?r1=1.102&r2=1.103)
        bufmgr.h (r1.121 -> r1.122)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/bufmgr.h?r1=1.121&r2=1.122)