On the log-addressing branch:  Add FSFS config file option to set
the block size in which we access rev / pack files.  For now, we
use it with aligned_seek only but it will soon also determine the
size of the blocks we parse.

* subversion/libsvn_fs_fs/fs.h
  (CONFIG_SECTION_IO,
   CONFIG_OPTION_BLOCK_SIZE): declare new config file elements
  (fs_fs_data_t): add block size

* subversion/libsvn_fs_fs/fs_fs.c
  (read_config): read the new option
  (write_config): extend config file template

* subversion/libsvn_fs_fs/cached_data.c
  (aligned_seek): take FS as parameter to select block size
  (open_and_seek_revision,
   open_and_seek_transaction,
   get_fs_id_at_offset,
   get_root_changes_offset,
   auto_read_diff_version,
   create_rep_state_body,
   read_delta_window,
   read_plain_window,
   get_contents,
   svn_fs_fs__get_changes): update callers


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/log-addressing@1509843 13f79535-47bb-0310-9956-ffa450edef68