Describe the bug
Envoy unable to communicate with Node Agent's SDS grpc server when the SDS grpc server's Serve() returns an error.

logs on Node Agent:
2019-01-15T22:47:58.874575Z error SDS grpc server for workload proxies failed to start: accept unix /var/run/sds/uds_path: use of closed network connection
2019-01-15T22:47:58.874608Z info SDS grpc server for workload proxies started
2019-01-15T22:47:58.874647Z info transport: http2Server.HandleStreams failed to read frame: read unix /var/run/sds/uds_path->@: use of closed network connection
2019-01-15T22:47:58.874658Z info transport: loopyWriter.run returning. connection error: desc = "transport is closing"
2019-01-15T22:47:58.874673Z info transport: http2Server.HandleStreams failed to read frame: read unix /var/run/sds/uds_path->@: use of closed network connection
2019-01-15T22:47:58.874694Z info transport: http2Server.HandleStreams failed to read frame: read unix /var/run/sds/uds_path->@: use of closed network connection
2019-01-15T22:47:58.874712Z info transport: http2Server.HandleStreams failed to read frame: read unix /var/run/sds/uds_path->@: use of closed network connection

Expected behavior
Envoy and Node Agent's SDS grpc server should be able to communicate successfully. If the SDS grpc server's Serve() returns an error, the communication should resume for subsequent connections.

Steps to reproduce the bug
Reproducible in a customer's environment.

Version
Latest istio release (2019-01-17).