osd: add OpRequest flag point when commit is sent

With writeahead journaling in particular, we can get requests that
stay in the queue for a long time even after the commit is sent to the
client while we are waiting for the transaction to apply to the fs.
Instead of showing up as 'waiting for subops', make it clear that the
client has gotten its reply and it is local state that is slow.

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