On the fsfs-format7 branch:  add P2L index access functions that allow
for returning single entries or even sub-items instead of whole page
descriptions.  This will be needed by the new index verification code
in fs_verify.

* subversion/libsvn_fs_fs/index.h
  (svn_fs_fs__p2l_entry_lookup,
   svn_fs_fs__p2l_item_lookup,
   svn_fs_fs__p2l_get_max_offset): declare new P2L index APIs

* subversion/libsvn_fs_fs/index.c
  (get_p2l_header): factored out from get_p2l_page_info
  (get_p2l_page_info): remainder adding cache access code
  (get_p2l_keys): new utility
  (p2l_index_lookup): factored out from svn_fs_fs__p2l_index_lookup
  (svn_fs_fs__p2l_index_lookup): remainder

  (compare_p2l_entry_offsets,
   get_p2l_entry_from_cached_page,
   p2l_entry_lookup_func): new partial cache access functions
  (p2l_entry_lookup): body of ...
  (svn_fs_fs__p2l_entry_lookup): ... this new API function

  (p2l_item_lookup_baton_t): new cache access baton type
  (p2l_item_lookup_func):  new partial cache access function
  (svn_fs_fs__p2l_item_lookup): implement new API function

  (p2l_get_max_offset_func): new partial cache access function
  (svn_fs_fs__p2l_get_max_offset): implement new API function


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1476660 13f79535-47bb-0310-9956-ffa450edef68