On the fsfs-format7 branch:  implement the 'block-read' feature.

Whenever we need to read data from the rev / pack file, we read
a whole block (64kB by default), parse all its contents and put
that into our caches.  The data that was actually requested will
be returned directly by block_read for most item types, i.e. no
extra read required.

The integration of this feature still needs some touch-up and
the block_read() function group needs docstrings.

* subversion/libsvn_fs_fs/cached_data.c
  (get_cached_node_revision_body): fuse with ...
  (get_node_revision_body): ... this one; clearly separate code
   paths for txn and committed data; invoke block-read
  (create_rep_state_body,
   svn_fs_fs__get_changes): invoke block-read
  (read_delta_window): ditto; optimize cache lookup sequence
  (block_read_rep_header,
   block_read_windows,
   block_read_contents,
   auto_select_stream,
   block_read_changes,
   block_read_noderev,
   block_read): implement the new feature

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1443210 13f79535-47bb-0310-9956-ffa450edef68