LU-3467 target: FLD to use unified request handler

LU-3467 target: FLD to use unified request handler

FLD is switched to unified target code, now it provides
own handler to be called by unified request handler.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I04fd6af9d11f2208ad220be25bb3245cc5edcccd
Reviewed-on: http://review.whamcloud.com/6764
Tested-by: Hudson
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>