$token : \Couchbase\MutationToken
— The optional, opaque mutation token related to updated document the environment.
Note that the mutation token is always NULL, unless they are explicitly enabled on the
connection string (`?fetch_mutation_tokens=true`), the server version is supported (>= 4.0.0)
and the mutation operation succeeded.
If set, it can be used for enhanced durability requirements, as well as optimized consistency
for N1QL queries.