Remove MessageTimestamp constructor from time_point

[mfontanini]

Relates to #128

[mfontanini]

@accelerated can you look into this one? I don't think we need that constructor, given the only one that constructs MessageTimestamps is cppkafka itself.

[accelerated]

See my comment in the Issue thread. I can move it to the header file, that was my oversight. I can also remove it, but I did make a note about that when I submitted the PR. Imho it's best to overload the MessageBuilder with a MessageTimestamp as argument to timeout().

[mfontanini]

I don't think a user should be able to create MessageTimestamp (right now it can't). The user shouldn't be able to specify a MessageTimestamp::TimestampType because when calling rd_kafka_producev that goes away: at that point it's just a millisecond count.

Given that nothing constructs MessageTimestamp besides the Message, there's no point in allowing creating them from time_point

[accelerated]

Ok, looks good to me! Merge away :)

[accelerated]

PS: A few minor observations:

• The MessageTimestamp class should ideally be in its own header.
• Its constructor should be private and Message class should be a friend. Then there's no confusion as to weather or not you can instantiate this class.

[mfontanini]

Yep, those are good ideas.