Reduce the size of an FSFS instance by using temporary pools
for temp. data during fs_open().

* subversion/libsvn_fs_fs/fs_fs.h
  (svn_fs_fs__initialize_caches): Document that the POOL shall be
                                  used for temporaries only.

* subversion/libsvn_fs_fs/caching.c
  (svn_fs_fs__initialize_caches): Fix the only place where we used
                                  POOL instead of FS->POOL for
                                  something with svn_fs_t lifetime.

* subversion/libsvn_fs_fs/fs.c
  (fs_serialized_init): Document POOL usage that we will now rely on.
  (fs_open): Use a SUBPOOL for anything temporary during FS init.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1588651 13f79535-47bb-0310-9956-ffa450edef68