pgsql: Rework subtransaction commit protocol for hot standby.

Log Message:
-----------
Rework subtransaction commit protocol for hot standby.

This patch eliminates the marking of subtransactions as SUBCOMMITTED in pg_clog
during their commit; instead they remain in-progress until main transaction
commit.  At main transaction commit, the commit protocol is atomic-by-page
instead of one transaction at a time.  To avoid a race condition with some
subtransactions appearing committed before others in the case where they span
more than one pg_clog page, we conserve the logic that marks them subcommitted
before marking the parent committed.

Simon Riggs with minor help from me

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        README (r1.11 -> r1.12)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/README?r1=1.11&r2=1.12)
        clog.c (r1.47 -> r1.48)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/clog.c?r1=1.47&r2=1.48)
        transam.c (r1.76 -> r1.77)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/transam.c?r1=1.76&r2=1.77)
        twophase.c (r1.45 -> r1.46)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c?r1=1.45&r2=1.46)
        xact.c (r1.265 -> r1.266)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c?r1=1.265&r2=1.266)
    pgsql/src/include/access:
        clog.h (r1.21 -> r1.22)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/clog.h?r1=1.21&r2=1.22)
        transam.h (r1.65 -> r1.66)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/transam.h?r1=1.65&r2=1.66)