On the log-addressing branch:  Store a checksum with each item
in the phys-to-log index.  Code to set and use will be added in
following commits.

Also, add precise file length information to that index.  We will
need that later during verification.

* subversion/libsvn_fs_fs/index.h
  (svn_fs_fs__p2l_entry_t): add checksum member
  (svn_fs_fs__p2l_get_max_offset): update docstring

* subversion/libsvn_fs_fs/index.c
  (p2l_header_t): add file size member
  (svn_fs_fs__p2l_index_create): store file size in header;
                                 add checksums to items
  (get_p2l_header): read file size as well
  (read_entry): read checksum as well
  (p2l_get_max_offset_func,
   svn_fs_fs__p2l_get_max_offset): return exact pack / rev file size

* subversion/libsvn_fs_fs/structure-indexes
  (Phys-to-log index): update

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/log-addressing@1516325 13f79535-47bb-0310-9956-ffa450edef68