LU-15317 osc: Add RPC to iotrace

LU-15317 osc: Add RPC to iotrace

Add RPCs to iotrace debugging.

To avoid creating too much debug output, this debug
ignores the possiblity that an RPC contains non-contiguous
extents.  Thus the eventual visualization will act as
though the RPC is a continuous whole.  I judge this to be
superior to the amount of log data and complexity of
capturing each extent separately.  If that level of detail
is needed, a higher debug level can be used.

Test-parameters: trivial

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I6fe416ba44be5572f130704ba9d3e9b85d09c656