Parsing revnode structures is the single most expensive (CPU-wise)
operation when reading data from FSFS.  Reduce the overhead.

* subversion/libsvn_fs_fs/low_level.c
  (read_header_block): use our fast hash plus the fact that we
                       already know the key sizes

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1544717 13f79535-47bb-0310-9956-ffa450edef68