Revision 1575428

In FSFS, remove padding from cache key structs to simplify handling.
Also, document why we want them to be 16 bytes whenever we can help it.

* subversion/libsvn_fs_fs/fs.h
  (pair_cache_key_t): Widen revision type to eliminate padding on
                      platforms with sizeof(long) != 8.  Document the
                      struct size rationale.
  (window_cache_key_t): Document the struct size rationale.