Customizable flush retried and timeout

[sash]

Is your feature request related to a problem? Please describe.
In case your kafka server is down or not reponsive you want a way to control the maximum time a message sent can take.

Describe the solution you'd like
Add configuration keys for both the retry count and the flush timeout to the configuration at a minimum or even better a way to configur them by using parameters to the producer's send method.

Describe alternatives you've considered
Retries are currently hardcoded to 10 and timeout is hardcoded at 1s. I tried to subclass the producer and override the flush method, but because of the use of private and not protected properties and methods, that is not an option!