Revision 1509342

On the log-addressing branch:  Bump FSFS format number and introduce
the new "addressing" option to fsfs format files.  Make that available
to our internal code through the new svn_fs_fs__use_log_addressing API.
IOW, we support mixed addressing repos from the beginning.

* subversion/libsvn_fs_fs/fs.h
  (SVN_FS_FS__FORMAT_NUMBER): bump to 7
  (SVN_FS_FS__MIN_LOG_ADDRESSING_FORMAT): declare new feature level
  (fs_fs_data_t): add start rev for logical addressing

* subversion/libsvn_fs_fs/fs_fs.c
  (read_format,
   svn_fs_fs__write_format): read / write the new addressing option
  (svn_fs_fs__open): update caller
  (upgrade_body): ditto; enable log addressing for the next shard, if any
  (svn_fs_fs__create): handle the generic compatibility option that 
                       will be introduced with FSX;
                       init addressing option depending on it
  (svn_fs_fs__info_format): update to accept f7

* subversion/libsvn_fs_fs/util.h
  (svn_fs_fs__use_log_addressing): declare new private API

* subversion/libsvn_fs_fs/util.c
  (svn_fs_fs__use_log_addressing): implement