[N1FTY] Close old gRPC ClientConns on refresh

As recommended here, it is advised to call close on gRPC.ClientConn whenever the application intends to not use it anymore ..

https://godoc.org/google.golang.org/grpc#Dial

To ensure resources are not leaked due to the stream returned, one of the following actions must be performed:

1. Call Close on the ClientConn.

2. Cancel the context provided.

3. Call RecvMsg until a non-nil error is returned. A protobuf-generated client-streaming RPC, for instance, might use the helper function CloseAndRecv (note that CloseSend does not Recv, therefore is not guaranteed to release all resources).

4. Receive a non-nil, non-io.EOF error from Header or SendMsg. 

On Refresh(..), N1FTY sets up a new set of ClientConns' to FTS servers but does not close the dereferenced ClientConns. This will need fixing.

I used this command to track the number of open gRPC connections ..

netstat -an | grep ESTABLISHED | grep -w 9201 | wc -l 

With some code instrumenting by forcing indexer refreshes continuously - I'm able to see that the gRPC connections count (available from the above command) grow over time. The fix for this issue ensures that the number remains constant despite continuous refresh.