LU-3467 target: generic hpreq handler in target

Make high-priority request handling generic. Each request handler
may initialize now not only generic handler but also high-priority
handler. Move specific OST hp callbacks to the OFD.

Remove rq_recovery_session from ptlrpc_request and use rq_session
always. That additional session was needed when recovery request
was copied, so the normal session might become freed. Now request
is not copied but referenced and only single session is enough.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: Iabf36d0828a86974bfe0638957f6018c919ac13b
Reviewed-on: http://review.whamcloud.com/7383
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>