iallocator: add ht-checking for the request

Currently, we only ht-check the result value from the iallocator, and
we send whatever we happen to check manually in the LUs that call the
iallocator.

This is not good, as we have to duplicate checks in many places, and
still we might miss checks. So we add add ht information to the
per-request variables. As the cluster data is built in one place, the
iallocator code itself (and is more consistent), I didn't add checks
to that too.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>