glusterfsd, libgfapi: destroy the temporary graphs constructed for comparison

* The new and the oldgraphs which have been constructed whenever there is
  a volfile change (either reconfigure of the existing graph or creating
  a new graph) for comparison should be freed. Otherwise frequent graph
  changes will lead to huge memory leak

Change-Id: I4faddb1aa9393b34cd2de6732e537a60f600026a
BUG: 948178
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/5388
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>