Add more dual-pool usage to the FSFS caching code.

* subversion/libsvn_fs_fs/cached_data.c
  (get_cached_window, read_delta_window): Switch to dual-pool and use
   a scratch pool for temporary allocations.
  (get_combined_window):  Pass an iterpool as scratch pool.
  (delta_read_next_window): Pass a scratch pool.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1604925 13f79535-47bb-0310-9956-ffa450edef68