Speed up svn_fs_fs__verify() by making verify_walker() reuse open rev
and pack file handles between iterations.  That gives a 3x speedup.

* subversion/libsvn_fs_fs/fs_fs.c
  (svn_fs_fs__verify): new optinal baton type
  (verify_walker): if baton is provided, reuse file handles between calls
  (svn_fs_fs__verify): provide verify_walker() with a baton


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1435749 13f79535-47bb-0310-9956-ffa450edef68