Sink connector ID configuration about cloudant-kafka-connector

[ricellis]

Rather than a sink connector configuration option I think providing (and documenting) a SMT that is able to insert the message key into the message value as an _id field is the way to go.

[ricellis]

Looking at the complexity that is coming out in #82 from trying to handle all the variations of inserting an _id into different types of messages and associated schema updates I think we need to re-think how we handle this.
One option would be to:

1. Modify the ConnectRecordMapper to check for the presence of a custom header on the record and use the value of that header as the ID.
2. Use the org.apache.kafka.connect.transforms.HeaderFrom to convert a key to the header, avoiding the need for us to have any custom SMT.

For the header, we would ensure the StringConverter was used as we only want strings for IDs. We would need to document the name of the header and the expectation that the value is a string. (this would cause errors for things that cannot convert cleanly to strings, inline with general Kafka behaviour on converters).

We would overwrite existing _id in the JSON blob before writing to Cloudant (with a warning).