Rebind read only

The guts of the changes to support a read-only / hot-standby mode for management.

The design minimizes code changes and lends itself to being adapted for multiple masters looking after different entities.

UPDATE - ready for review and merging when appropriate

I have recently completed:

* Programmatic means of forcing transition from `FAILURE` to `STANDBY` or `HOT_STANDBY` (or `TERMINATED`) and from `MASTER` to `STANDBY`, `HOT_STANDBY`, or `TERMINATED`, and setting priority
* New `HOT_STANDBY` mode and `readOnlyRebind`
* Have a property determining whether default is `STANDBY` or `HOT_STANDBY`
* Failure on `HOT_STANDBY` moves to `FAILURE` mode
* Support `BrooklynObject` replacement on subsequent rebind (and unmanagement when items removed)
* Ensure `BrooklynObject` unmanagement and cleanup on demotion (this probably doesn't happen)

We still need TO DO the following:

* Prevent activity for policies, feeds, enrichers, etc (possibly by promoting `suspend`/`resume` to `EntityAdjunct` and `AbstractEntityAdjunct` and in the latter checking the RO status of the entity; this won't catch everything but should catch most and make it easier to refactor those things which attempt a scheduled task inside `setEntity` ... that's not quite the answer, but in that direction I feel)
* GUI/REST support to, at minimum, allow operators to access (after a warning probably) standby and failed servers, to display rebind problems, to set HA priority and force HA state changes -- this will allow us to design a sensible `BrooklynCluster` with failover!

But these can proceed independently; what is in this PR now feels like a complete package.
good add'l assertion.  i've added it.

this test just tests the initial custom value. the following test asserts subsequent changes and more complex ones.  i've renamed both to make that clearer, thx.