Finalize work on svn_fs_fs__get_stats API.  Add cancellation support
to the API and make svnfsfs actually provide a standard cancellation
callback.

* subversion/svnfsfs/svnfsfs.h
  (check_cancel): Declare global cancellation callback to be accessible
                  to all sub-commands.

* subversion/svnfsfs/svnfsfs.c
  (check_cancel): New global cancellation callback. Copied over from
                  svnadmin.c

* subversion/svnfsfs/stats-cmd.c
  (query_t): Tweak docstrings and add cancellation callback members.
  (read_phys_pack_file,
   read_phys_revision_file,
   read_log_rev_or_packfile): Invoke the cancellation callback once
                              in a while.
  (create_query): Store cancellation callbacks in the query struct.
  (svn_fs_fs__get_stats): Add cancellation support.
  (subcommand__stats): Update API caller.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632694 13f79535-47bb-0310-9956-ffa450edef68