libceph: init sent and completed when starting

The rbd code has a need to be able to restart an osd request that
has already been started and completed once before.  This currently
doesn't work right because the osd client code assumes an osd
request will be started exactly once.

Specifically, the r_sent field records the osd incarnation at the
time the request was sent to that osd.  If that's non-zero, the
message won't get re-mapped to a target osd properly, and won't be
put on the unsafe requests list the first time it's sent as it
should.  The r_sent field is currently never cleared.

Similarly, the r_completed field is used for lingering requests to
avoid calling the callback function every time the osd client
re-sends the request on behalf of its initiator.  This field
is also not cleared.

Each osd request passes through ceph_osdc_start_request() when
responsibility for the request is handed over to the osd client for
completion.  We can safely zero the r_sent and r_completed fields
there each time a request gets started.

Signed-off-by: Alex Elder <elder@inktank.com>