Fix IPAddress and CIDR type decorators

Use load_dialect_impl() user hook instead of calling
with_variant() on a type implementation in IPAddress
and CIDR type decorators. This makes this code work
with SQLAlchemy 0.8.x versions.

Fixes bug 1200231

Change-Id: Ib0eb582f472116bda817d3e35e26888dc116ad58
Merge "Fix IPAddress and CIDR type decorators"