Add consumer flow control

[Gsantomaggio]

Add Consumer Flow control.
Configure the way to request the credits with the following parameters:

public enum ConsumerFlowStrategy
{
    CreditsBeforeParseChunk,
    CreditsAfterParseChunk,
    ConsumerCredits
}
var consumerConfig = new RawConsumerConfig(stream)
 {
     FlowControl = new FlowControl() { 
        Strategy = ConsumerFlowStrategy.CreditsBeforeParseChunk
     },

Add rawConsumer.Credits to manually request Credits

Closes #420