When cluster map contains no VBuckets handle gracefully

When a bucket is created (via REST API for example) and a client immediately tries to bootstrap of it, the initial cluster map may contain an empty VBucketMap. If a client tries to use the empty VBucketMap, it will throw KeyNotFoundExceptions, but try to re-bootstrap.

• The correct behavior should be to back for n seconds and then try to bootstrap again.
• Alternatively, the client can raise an exception with the cause and a message indicating that the App code needs to back off and try again in n seconds.