Envoy ALS message rotation on multiple streams

This issue proposes to introduce connection re-establishment in the Envoy ALS gRPC client. In the current implementation, the connection between Envoy and the receiver survives for so long that scaling out the receiver does not make sense; in order to perform load balancing of the Envoy ALS receiver, the Async gRPC client of Envoy needs to re-establish the connection at regular intervals or at some To load balance the Envoy ALS receiver, the Async gRPC client of Envoy needs to re-establish the connection at regular intervals or according to some other criteria. When no connection is established, logs are watermark buffered and flushed as needed.

cc. @lizan