Replace Map<String,String[]> with serializable Metadata

[jnioche]

Having a wrapper for Metadata would be a bit more elegant than moving the Map<String,String[]> around.
This initial implementation adds Metadata and removes the KeyValues utility. Later on we can replace all occurrences of Map<String,String[]> and systematically use Metadata instead.

[jnioche]

@GuiForget any thoughts on this?

[GuiForget]

I would check for null and make a copy of the map

[GuiForget]

Ok given that for now you use the Metadata class just as a wrapper, making a copy isn't possible. Checking for null is probably a good idea still

[jnioche]

this is temporary indeed. Am now throwing an exception when the value is null.

[jnioche]

thanks for your comments Gui

[jnioche]

Quite a large change, I think we should release 0.4 first then merge this.
@GuiForget @DigitalPebble/committers-crawler

[jakekdodd]

Now that we've released 0.4, should we make this priority 1? I think several other issues are affected by the new metadata scheme

[GuiForget]

Yes yes yes!!! 💃

My only concern is should we just go further and replace the use of Map<String, String[]> with Metadata class right away?

[jnioche]

+1 to make it top priority. I will take care of this.

replace the use of Map with Metadata class right away

that's what the old PR was attempting to do, might as well do it all in one go.

Can't output it from a bolt and not get the following bolt to handle them, can't we?

[jnioche]

Opened a new PR #78