Branch b1_6

Disable client page cache checksumming by default. This does not affect
the wire checksumming, and only adds a lot of CPU overhead that is mostly
useful only for debugging rare client memory corruption vs. the more
common network transfer corruption. It can be enabled at runtime via
"echo 1 > $LPROC/llite/*/checksum_pages".
b=13579
i=scjody
i=nathan