DurabilityInvalidLevel in transactions cleanup

Couchbase Server resposne to transaction cleanup request with "DurabilityInvalidLevel." Accoridng to Jim, this means the client encoded the durability level as outside the range [1,3] or specified an invalid timeout.

The following log message shoes the durability was represented as `durability=Optional[NONE]` instead of `durability=Optional.empty()`.

MemcacheProtocol.mutationFlexibleExtras() doesn't expect this – it only checks for empty optional, and is probably encoding `0` as the durability level, which the server rejects.

We could take a few different approaches. Simplest might be to add a check for `durability=Optional[NONE]` in mutationFlexibleExtras(), and treat it the same as Optional.empty().

Another possible solution would be to re-evaluate whether we need an Optional at all – maybe DurabilityLevel.NONE is sufficient to indicate an absence of synchronous durability requirement. Might need to see how this interacts with legacy durability (persistTo, replicateTo).