Memcached buckets try to send Observe operations

When adding additional testing for Memecached, it was found that the MemcachedBucket tries to submit Observe operations to the cluster for Exists and ExistsAsync IBucket requests.

Memcached bucket do not support Observe operations because they do not support VBuckets and when the client tries to create the internal Observe operation throws a NullException during the Write method because the VBucket property has not been set.

There were also existing tests that were passing, this is because it utilises a Couchbase Bucket instead of a Memcached bucket.

All MemcachedBucket Exists and ExistsAsync variants should return NotSupportedExceptions and not attempt to send Observe operations that fail.