Add retry policy to grpc connect helper

[smacker]

Analyzers using sdk should retry to connect to data server if the connection is broken.

Ref: src-d/style-analyzer#509

[smacker]

It's impossible to do on connect level. We can change default timeouts but they seem very reasonable.

For go we can workaround it using ResetConnectBackoff when we receive an event. I added documentation about it.
For python I can't reproduce the error with new grpcio. Most probably it works a little bit differrent from go implementation and the current error was caused by grpc/grpc#15983