This resolves an issue where the original sort order sent back by ES was
lost for point series / vislib visualizations with split series. This
was due to the way the point series agg response handler generated
series data, only filling in series values as it encountered them
bucket-by-bucket, rather than first looking at all x-values and ordering
them consistently within each series.

With this change, when a series is first created in the `agg_response`, it
will first look at all results, preserving the x-value sort order. Then
when creating new series, it will instantiate a zero-filled array with
the correctly ordered x axis values, filling it in with the real values
as it encounters them.

This duplicates some of the work done in the vislib `zero_injection`
component, which can likely be cleaned up further, or possibly removed
entirely.