add support for persisting to object store

READY FOR REVIEW - now includes HA, CLI.

basis is to refactor file-based support to use a common PersistenceObjectStore interface backed by either filesystem or jclouds blobstore (running w softlayer only so far, but using jclouds).

to run integration tests against SL, set up a named location such as the following:

    brooklyn.location.named.softlayer-objectstore-amsterdam-1=jclouds:swift:https://ams01.objectstorage.softlayer.net/auth/v1.0
    brooklyn.location.named.softlayer-objectstore-amsterdam-1.identity=IBMOS1234:yourname
    brooklyn.location.named.softlayer-objectstore-amsterdam-1.credential=....

have completed a big rewrite of the `StoreObjectAccessor` classes to resolve these issues, going with a sync approach where one could layer on an async wrapper.  much simpler i think, with a locking wrapper doing the RW-lock.