ra_serf: Fix high memory usage running svnsync.

* subversion/libsvn_ra_serf/replay.c
  (push_state): Use the file_pool for allocating file specific data.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x-svnsync-serf-memory@1515249 13f79535-47bb-0310-9956-ffa450edef68