Move FSFS index rewrite functions from svnfsfs to index.* and give
it them a two-pool signature.

This prepares the switch to a partly generated revision zero.

* tools/server-side/svnfsfs/load-index-cmd.c
  (calc_fnv1,
   write_p2l_index,
   compare_p2l_entry_revision,
   write_l2p_index): Move this code from here to index.c. 
 (load_index): Call the new internal API instead.

* subversion/libsvn_fs_fs/index.c
  (calc_fnv1,
   compare_p2l_entry_revision): Code moved over from load-index-cmd.c
  (svn_fs_fs__l2p_index_from_p2l_entries): Former write_l2p_index with
                                           modified signature.
  (svn_fs_fs__p2l_index_from_p2l_entries): Same for write_p2l_index.

* subversion/libsvn_fs_fs/index.h
  (svn_fs_fs__l2p_index_from_p2l_entries,
   svn_fs_fs__p2l_index_from_p2l_entries): Declare new internal API.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606528 13f79535-47bb-0310-9956-ffa450edef68