Implement DrainAgent master/operator call with associated registry actions

message Call {
  enum Type {

    . . .

    DRAIN_AGENT = 37;
    DEACTIVATE_AGENT = 38;
    REACTIVATE_AGENT = 39;
  }

  . . .

  message DrainAgents {
    message DrainConfig {
      required AgentID agent = 1;

      // The duration after which the agent should complete draining.
      // If tasks are still running after this time, they will
      // be forcefully terminated.
      optional Duration max_grace_period = 2;

      // Whether or not this agent will be removed permanently
      // from the cluster when draining is complete.
      optional bool destructive = 3 [default = false];
    }

    repeated DrainConfig drain_config = 1;
  }

  message DeactivateAgents {
    repeated AgentID agents = 1;
  }

  message ReactivateAgents {
    repeated AgentID agents = 1;
  }
}

message Registry {

  . . .

  message Slave {
    . . .

    optional DrainInfo drain_info = 2;
  }

  . . .

  message UnreachableSlave {
    . . .

    optional DrainInfo drain_info = 3;
  }
}