Add a pool member to the internal representation of FSX IDs and
thus allow the ID vtable functions to perform actual data lookups
in the future.  The pool is the one used to allocate the ID.

This is a first step towards making FSX IDs leaner and smarter.  

* subversion/libsvn_fs_x/id.h
  (svn_fs_x__id_deserialize):  Add POOL parameter that is already
                               available to calling functions.

* subversion/libsvn_fs_x/id.c
  (fs_x__id_t): Extend internal ID representation.
  (svn_fs_x__id_eq): Adjust the amount of data to compare.
  (svn_fs_x__id_txn_create_root,
   svn_fs_x__id_create_root,
   svn_fs_x__id_txn_create,
   svn_fs_x__id_rev_create): Initialize the new struct member.
  (svn_fs_x__id_copy): Ditto, plus use a more efficient duplication func.
  (svn_fs_x__id_parse,
   svn_fs_x__id_deserialize): Initialize the new struct member.

* subversion/libsvn_fs_x/temp_serializer.h
  (svn_fs_x__noderev_deserialize): Add POOL parameter as pass-through.

* subversion/libsvn_fs_x/temp_serializer.c
  (deserialize_dir,
   svn_fs_x__noderev_deserialize,
   svn_fs_x__deserialize_id,
   svn_fs_x__deserialize_node_revision,
   svn_fs_x__extract_dir_entry,
   deserialize_change,
   svn_fs_x__deserialize_changes): Provide everyone who needs it with
                                   a POOL parameter.

* subversion/libsvn_fs_x/dag.c
  (svn_fs_x__dag_deserialize): Update API caller.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1548823 13f79535-47bb-0310-9956-ffa450edef68