couchbase: add counter operations increment and decrement

[sapk]

This PR add atomic counter operations support for Couchbase processor.

This is backward compatible as they are new operations.

Note: This might conflict with #2709 but I can rebase over the first to be merged.

[CLAassistant]

All committers have signed the CLA.

[sapk]

I updated the PR to have the ttl support also on increment and decrement operations since #3825 was merged.

I also change to require and enforce that content is a valid integer. Previously any invalid or empty value set the delta to zero meaning a noop but it had the bad design of silencing error.
If wanted we can set and document a default value for delta if content is empty. Either 1 to increment or decrement by 1 by default or 0 to be an noop by default (simply retrieve the existing value).

Also I simply return the error in process batch but maybe I could be more subtle and attach the error on the message and skip adding it to the ops list.

[peczenyj]

At least this feature is available in the other implementation