rgw: return the update status on sync requests

rgw: return the update status on sync requests

Add new STATUS_APPLIED, then specify the RGWX_UPDATE_STATUS header
based on that return code when doing metadata puts.
Add a send_response() function to RGWOp_Metadata_Put in order to
support sending back our new headers. Move the translation from
STATUS_NO_APPLY from set_req_state_err() to this function, so we
can turn different sync results into failures if necessary elsewhere.

Signed-off-by: Greg Farnum <greg@inktank.com>