couchbase: add CAS support

[sapk]

Set and use Couchbase CAS in metadata couchbase_cas to prevent write conflict.
If an entry is modified by an other process between read and write it will return an error if CAS is mismatched.

This change should be transparent and prevent data loss by default.
This check can be disabled by clearing the CAS value if needed.

[CLAassistant]

All committers have signed the CLA.