synctask: implement setuid-like SYNCTASK_SETID()

synctasks can now call SYNCTASK_SETID(uid,gid) to set the effective
uid/gid of the frame with which the FOP will be performed.

Once called, the uid/gid is set either till the end of the synctask
or till the next call of SYNCTASK_SETID()

Change-Id: I7eb74f7c473099bcae39310d2ab353d58f8eb2ba
BUG: 884597
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4269
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Shishir Gowda <sgowda@redhat.com>