Use one Augeas handler per resource

This PR aims to limit the amount of Augeas handlers used per resource.

This is the first step to implement what I described in https://groups.google.com/d/msg/puppet-dev/kWpTdXSP-L4/LeGg3MTjqGsJ .

Known issues:
The current PR still fails on 2 tests, and Augeas#close is never called (except in the case of an exception).