LU-15317 llite: Add COMPLETED iotrace messages

LU-15317 llite: Add COMPLETED iotrace messages

It's very useful to see how long an I/O call took.  There
are other ways to do this, but the goal is for iotrace to
provide all necessary information for basic I/O performance
analysis, so we add COMPLETED messages to iotrace.

Test-Parameters: trivial

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I17f52ebc87a31d5ba34f63dc8b6a279e83cd10ef
Reviewed-on: https://review.whamcloud.com/46484
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>