MemcachedBucket incorrectly uses observe cmd for Exists and ExistsAsync

Memcached buckets do not support the observe (0x92) cmd, however the implementation provided uses the observe cmd, thus always returning UnknownCommand (0x0081). The unit tests written for that method were using the wrong bucket type, therefore succeeding.

These either need to throw NotSupportExceptions or perhaps rewritten with another cmd so that they work.