Introduce a pool wrapper for the iteration pool idiom in the C++HL APR wrapper.

* subversion/bindings/cxxhl/src/aprwrap/pool.hpp
  (IterationPool): New class; a pool wrapper that hides its API.
  (Pool::Pool): Make non-default constructor explicit and accept a non-const
   reference to the parent pool instead of a const pointer.
  (Pool::Iteration): New nested class; a proxy for the pool in IterationPool.
* subversion/bindings/cxxhl/src/exception.cpp
  (Error::compile_messages): Use IterationPool and Pool::Iteration in the loop.
* subversion/bindings/cxxhl/tests/test_aprwrap.cpp
  (CreateSubpool): Adapt to changed pool constructor signature.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507217 13f79535-47bb-0310-9956-ffa450edef68