Adds AttributeSensor.persistenceMode

- If set to NONE then that attribute’s value will not be persisted, so will
  not be available on rebind.
- Adds Sensors.builder() to avoid too much overloading.
BROOKLYN-102: Fix MongoDB rebind

- Declare STATUS_BSON as transient, so won’t be persisted
  (because that value failed to rebind)
This closes #397