Revert "librbd: ensure header is up to date after initial read"

Using assert version for linger ops doesn't work with retries,
since the version will change after the first send.
This reverts commit e1776809031c6dad441cfb2b9fac9612720b9083.

Conflicts:

	qa/workunits/rbd/watch_correct_version.sh
librbd: establish watch before reading header

This eliminates a window in which a race could occur when we have an
image open but no watch established. The previous fix (using
assert_version) did not work well with resend operations.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge remote branch 'origin/wip-rbd-watch'

Reviewed-by: Dan Mick <dan.mick@inktank.com>