A metric aggregation that aggregates a set of points as
a GeoJSON LineString ordered by some sort parameter.

A `geo_line` aggregation request would specify a `geo_point` field, as well
as a `sort` field. `geo_point` represents the values used in the LineString,
while the `sort` values will be used as the total ordering of the points.

the `sort` field would support any numeric field, including date.

```
{
	"query": {
		"bool": {
			"must": [
				{ "term": { "person": "004" } },
				{ "term": { "trajectory": "20090131002206.plt" } }
			]
		}
	},
	"aggs": {
		"make_line": {
			"geo_line": {
				"point": {"field": "location"},
				"sort": { "field": "timestamp" },
                                "include_sort": true,
                                "sort_order": "desc",
                                "size": 15
			}
		}
	}
}
```

```
{
    "took": 21,
    "timed_out": false,
    "_shards": {...},
    "hits": {...},
    "aggregations": {
        "make_line": {
            "type": "LineString",
            "coordinates": [
                [
                    121.52926194481552,
                    38.92878997139633
                ],
                [
                    121.52922699227929,
                    38.92876998055726
                ],
             ]
        }
    }
}
```

Due to the cardinality of points, an initial max of 10k points
will be used. This should support many use-cases.

One solution to overcome this limitation is to keep a PriorityQueue of
points, and simplifying the line once it hits this max. If simplifying
makes sense, it may be a nice option, in general. The ability to use a parameter
to specify how aggressive one wants to simplify. This parameter could be
the number of points. Example algorithm one could use with a PriorityQueue:
https://bost.ocks.org/mike/simplify/. This would still require O(m) space, where m
is the number of points returned. And would also require heapifying triangles
sorted by their areas, which would be O(log(m)) operations. Since sorting is done,
anyways, simplifying would still be a O(n log(m)) operation, where n is the total number
of points to filter........... something to explore

closes elastic#41649