[FLINK-15952] Replace the usage of a raw byte[] for keys in MultiplexedState

[igalshilman]

This PR address the problem, that when using Flink's heap state backend keys to a MapState can not be of a primitive array type.
This PR introduces a new Protobuf message to be used as a key in a multiplexed state context.

message MultiplexedStateKey {
    string function_namespace = 1;
    string function_type = 2;
    string state_name = 3;
    repeated bytes user_keys = 4;
}

[tzulitai]

+1, LGTM. Thanks for fixing this.

I'll give a local run with the greeter example, and then merge this if Travis agrees as well 👌

[tzulitai]

Local run of the greeter example works, and Travis is green as well.
Merging ...