Add a GetFieldMapping API

[bleskes]

This new API allows to get the mapping for a specific set of fields rather than get the whole index mapping and traverse it.
The fields to be retrieved can be specified by their full path, index name and field name and will be resolved in this order.
In case multiple field match, the first one will be returned.

Since we are now generating the output (rather then fall back to the stored mapping), you can specify include_defaults=true on the request to have default values returned.

Closes #3941

[kimchy]

maybe we can write this field as "raw", see XContentHelper#rawField, so no need to convert it to a map?

[bleskes]

Done. Required moving the source to a ByteReference which is ok imo.

[spinscale]

I am in the same boat as luca with putting if (includeInDefaults) in front of everything inside of toXContent() in the mappers is adding up to complexity. I am in favor of having a toXContentEverything() and the old toXContent() methods or at least having sth like if (includeInDefaults) { } else { } in toXContent() - even at the price of duplicating code I think this makes it way more readable.

Apart from that: from functionality point of view this is a huge +1

[bleskes]

@javanna @spinscale thx for the review. Going back to the point you made about potentially having a doXContentBodyWithDefaults() method that will remove the need for includeDefaults tests. I see your point but to me it's a choice between evils. The down side of such a method is that when we add something we need to remember to add it both places and the code doesn't remind us to (if you don't notice there is another method, you'd miss it). Same goes for bug fixes in existing code. Unless someone feels strongly about it, my choice goes to leave it as is.

[kimchy]

on the fence on this as well, leaning towards the current solution, where the serialization of a single mapping attribute is done in a single place (compared to over several places).

[javanna]

I was more leaning towards a common method that does the if, as mentioned above, but there certainly are downsides to it as well (too many variations needed is the main one). I think I prefer those downsides anyway over the repeated if, but it's hard to tell without writing the code I guess... :)

[javanna]

private static too?