Support null value messages

Currently the producer cannot send a null value message, which is used as a tombstone for a specific key on a compacted topic.

In addition, the consumer cannot differ empty values from null values, because all these values are presented as empty std::strings.

To solve this issue, we might need to add the following method to MessageBuilder:

MessageBuilder& setNullValue();

The method above should set the following field in MessageMetadata:

message MessageMetadata {
    // ...
    // Indicate if the message payload value is set
    optional bool null_value = 25 [default = false]

Then we can add a method to Message to indicate if the message has null value

bool isNullValue() const;

@BewareMyPower can you assign this issue to me?

OK