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.

[gregfurman]

Thanks for this great contribution @sapk! I've left a couple of comments and suggestions. Mostly some minor grammar and spelling fixes. Also, feel free to discard any nit comments.

I'm curious whether we should be allowing for these CAS checks to be toggled via the config instead of making a user clear out metadata fields with bloblang. Not super familiar with Couchbase though so maybe it's more desirable to attach these values by default (like what your PR is currently doing). Let me know your thoughts on this.

[sapk]

It should be good now.

[gregfurman]

@sapk Thanks for the follow-up! I added one small suggestion for the docs to no longer include that deleted() approach of disabling things. If you're happy with that, just regen the docs and we can merge this in.

[gregfurman]

LGTM! 👍