On the thunder branch: Extend THUNDER-awareness to the L2P index in format7.
The reverse (P2L) index will be indirectly covered by block_read in a
following commit.

* subversion/libsvn_fs_fs/util.h
  (svn_fs_fs__thundered_cache_get_partial): Another THUNDER-handling wrapper
                                            around our cache accessors.

* subversion/libsvn_fs_fs/util.c
  (svn_fs_fs__thundered_cache_get_partial): Implement it.

* subversion/libsvn_fs_fs/index.c
  (get_l2p_page_info,
   l2p_index_lookup): Use THUNDER-aware cache / data access.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/thunder@1586554 13f79535-47bb-0310-9956-ffa450edef68