Allow setting document ID from message field

Add an optional config property that identifies a field of the message to use as the Couchbase document ID. If this config value is not specified or the referenced field is not present in the message, fall back to using the message key as the ID, or if that is not possible, use topic/partition/offset.

Add a second optional configuration property that determines whether the document ID field should be removed from the document before persisting to Couchbase.