pgsql: Tune GetSnapshotData() during Hot Standby by avoiding loop

Log Message:
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Tune GetSnapshotData() during Hot Standby by avoiding loop
through normal backends. Makes code clearer also, since we
avoid various Assert()s. Performance of snapshots taken
during recovery no longer depends upon number of read-only
backends.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xlog.c (r1.397 -> r1.398)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.397&r2=1.398)
    pgsql/src/backend/storage/ipc:
        procarray.c (r1.62 -> r1.63)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c?r1=1.62&r2=1.63)
    pgsql/src/include/access:
        xlog.h (r1.106 -> r1.107)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog.h?r1=1.106&r2=1.107)