Optimize the mergeinfo_count checking code in FSX.  Add a partial cache
accessor for mergeinfo_counts in noderevs containers.  Plus provide the
infrastructure handing the info through.

* subversion/libsvn_fs_x/noderevs.h
  (svn_fs_x__mergeinfo_count_get_func): declare new partial accessor

* subversion/libsvn_fs_x/noderevs.c
  (svn_fs_x__mergeinfo_count_get_func): implement

* subversion/libsvn_fs_x/cached_data.h
  (svn_fs_x__get_mergeinfo_count): declare new data access function

* subversion/libsvn_fs_x/cached_data.c
  (svn_fs_x__get_mergeinfo_count): implement

* subversion/libsvn_fs_x/tree.c
  (verify_node): use the new mergeinfo accessor to speed up


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1520437 13f79535-47bb-0310-9956-ffa450edef68