cassandra rebind

Improvements to our general rebind code, motivated by getting things like Cassandra to persist/rebind correctly.

This has so far just been tested with integration-tests, rather than manual/real brooklyn launcher.

Big things still to do include:
* CLI options for BrooklynLauncher, to configure persistence
* Rebinding locations promptly (rather than piggy-backing on persistence of the entity that references it). This will be fixed by making locations into entities.
* Task persistence.