Implement MemoryNameSystemClient

• Client class that has a proxy to the MNS and can be used to construct (single) Memory Clients as well as fetch the data for any Memory IDs.
• Like in DNS, it stores a map from memory names to proxies which is used if the respective entry exists. Otherwise, it queries the MNS. If the memory name is unknown to the MNS, an exception is thrown.
• Provides the other interface functions of the MNS.
• Can be used to query arbitrary Memory IDs (resolving the memory name, then sending a query to that memory, returning the result)