In FSFS, transaction layer, rename a few pool arguments of single pool
functions in accordance to the two-pool paradigm.

This is not systematic, yet, but to reduce the size of following patches.

* subversion/libsvn_fs_fs/transaction.c
  (store_sha1_rep_mapping
   write_container_rep,
   write_container_delta_rep): Pools for temporaries shall be SCRATCH_POOLs.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1605065 13f79535-47bb-0310-9956-ffa450edef68