Should use the cas value

[janober]

This is related to #8. Where the cas is returned on document create with the id.

Like written there, the documents should always be exactly the same (no matter if query or create). However would be good if the cas would always be returned in for example "_cas", "_metadata.cas" or whatever. On save the connector should then use the cas when present to see if it matches with the current one and only update when they match. If not it should error with "cas mismatch" so that the application can change the data again based on the now current-document.

This is very important because there is no locking and data will else be overwritten all the time.

[janober]

Started to implement it yesterday but did run into problems. Was unable to use the cas value which I queried from N1QL. The normal node-api returns an object with 2 values while N1QL returns only a number. After checking back with the Couchbase-Community it seems there is currently no solution for that but they logged it as a bug and fix it hopefully soon:
https://issues.couchbase.com/browse/JSCBC-196

Will finish my implementation once it is fixed.