Couchbase sync gateway

Resolves jira task BROOKLYN-9
An alternative to using the HttpFeed would be to use a `brooklyn.util.repeat.Repeater` to poll until success/fail.

It looks like `BUCKET_CREATION_IN_PROGRESS` is just used in this method, and unneccessarily by the caller of the createBuckets method (because it sequentially calls createBucket, which waits for bucket-creation to complete before returning).

Using the attribute to control synchronization feels dangerous - e.g. on brooklyn restart would anything reset it to `false`. Also, there is a race if two effectors execute concurrently - they both wait for it to be false, and then both set it to true. Better would be to use standard Java synchronization mechanisms in createBucket.