Always return the error of "GrpcAPI(Status { code: Unavailable ..."

[pingyu]

Reproduce step:

1. Startup cluster.
2. Perform workload.
3. Restart the cluster (the cluster is startup by tiup playground and the IP address of stores are changed after restart).

Then on every retry, the client always return the error of "GrpcAPI(Status { code: Unavailable ..." when get value of key using Snapshot::get:

Mar 16 16:32:06.441 ERRO ... ClientError("get: key ..., err: GrpcAPI(Status { code: Unavailable, message: \"error trying to connect: tcp connect error: Connection refused (os error 111)\", source: Some(tonic::transport::Error(Transport, hyper::Error(Connect, ConnectError(\"tcp connect error\", Os { code: 111, kind: ConnectionRefused, message: \"Connection refused\" })))) }))")
Mar 16 16:32:10.998 ERRO ... ClientError("get: key ..., err: GrpcAPI(Status { code: Unavailable, message: \"error trying to connect: tcp connect error: Connection refused (os error 111)\", source: Some(tonic::transport::Error(Transport, hyper::Error(Connect, ConnectError(\"tcp connect error\", Os { code: 111, kind: ConnectionRefused, message: \"Connection refused\" })))) }))")
Mar 16 16:32:15.553 ERRO ... ClientError("get: key ..., err: GrpcAPI(Status { code: Unavailable, message: \"error trying to connect: tcp connect error: Connection refused (os error 111)\", source: Some(tonic::transport::Error(Transport, hyper::Error(Connect, ConnectError(\"tcp connect error\", Os { code: 111, kind: ConnectionRefused, message: \"Connection refused\" })))) }))")
Mar 16 16:32:20.111 ERRO ... ClientError("get: key ..., err: GrpcAPI(Status { code: Unavailable, message: \"error trying to connect: tcp connect error: Connection refused (os error 111)\", source: Some(tonic::transport::Error(Transport, hyper::Error(Connect, ConnectError(\"tcp connect error\", Os { code: 111, kind: ConnectionRefused, message: \"Connection refused\" })))) }))")
Mar 16 16:32:24.667 ERRO ... ClientError("get: key ..., err: GrpcAPI(Status { code: Unavailable, message: \"error trying to connect: tcp connect error: Connection refused (os error 111)\", source: Some(tonic::transport::Error(Transport, hyper::Error(Connect, ConnectError(\"tcp connect error\", Os { code: 111, kind: ConnectionRefused, message: \"Connection refused\" })))) }))")
Mar 16 16:32:29.226 ERRO ... ClientError("get: key ..., err: GrpcAPI(Status { code: Unavailable, message: \"error trying to connect: tcp connect error: Connection refused (os error 111)\", source: Some(tonic::transport::Error(Transport, hyper::Error(Connect, ConnectError(\"tcp connect error\", Os { code: 111, kind: ConnectionRefused, message: \"Connection refused\" })))) }))")
Mar 16 16:32:33.783 ERRO ... ClientError("get: key ..., err: GrpcAPI(Status { code: Unavailable, message: \"error trying to connect: tcp connect error: Connection refused (os error 111)\", source: Some(tonic::transport::Error(Transport, hyper::Error(Connect, ConnectError(\"tcp connect error\", Os { code: 111, kind: ConnectionRefused, message: \"Connection refused\" })))) }))")

[pingyu]

Close by #484