osd: move queue checks into enqueue_op, kill _handle_ helpers

osd: move queue checks into enqueue_op, kill _handle_ helpers

This simplifies things, and renames the checks to make it clear that we are
doing validation checks only, with no side-effects allowed.

Also move some checks into the parent handle_op() to further simplify the
(re)queue checks.

Signed-off-by: Sage Weil <sage.weil@dreamhost.com>