pgsql: Replace ARC cache management algorithm with the similar but

Log Message:
-----------
Replace ARC cache management algorithm with the similar but slightly
simpler 2Q algorithm, to avoid possible problems with the pending patent
on ARC.  Testing so far suggests that there is little if any performance
loss from doing this.

Note that this patch is going into the 8.0 branch only; a much more
extensive revision is planned for HEAD.

Tags:
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REL8_0_STABLE

Modified Files:
--------------
    pgsql/doc/src/sgml:
        runtime.sgml (r1.301.4.2 -> r1.301.4.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.301.4.2&r2=1.301.4.3)
    pgsql/src/backend/storage/buffer:
        README (r1.7 -> r1.7.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/README.diff?r1=1.7&r2=1.7.4.1)
        freelist.c (r1.49.4.1 -> r1.49.4.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/freelist.c.diff?r1=1.49.4.1&r2=1.49.4.2)