client: remove O_LAZY

The once-upon-a-time unique O_LAZY value I chose forever ago is now
O_NOATIME, which means that some clients are choosing relaxed
consistency without meaning to.

It is highly unlikely that a real O_LAZY will ever exist, and we can
select it in the ceph case with the ioctl or libcephfs call, so drop
any support for doing this via open(2) flags.

Update doc/lazy_posix.txt file re: lazy io.

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>