Reconcile should perform a diff with Azure

At the moment we are doing a unilateral PUT of each resource when we reconcile; this works but has some drawbacks.

* Users with many resources have run up against throttling issues (e.g. #2591, #2643)
* Users have raised bugs because they thought the operator should only PUT when there's a change required (e.g. #2590)

We should diff the current state of the resource with the spec and only do a PUT where required (see #2600 for potential design).

Follow-up to #1491 as much of that has been implemented already.

No files or tests are named. Start by reading the existing work referenced in #1491 and the potential design in #2600, then trace the reconciliation path that currently PUTs each resource; done means unchanged resources are not PUT and changed resources still reconcile correctly.

Written by the indexing model from the issue text.