Before this change we inspected the index when optimizing
`date_histogram` aggregations, precalculating the divisions for the
buckets for the entire range of dates on the index so long as there
aren't a ton of these buckets. This works very well when you query all
of the dates in the index which is quite common - after all, folks
frequently want to query a week of data and have daily indices.

But it doesn't work as well when the index is much larger than the
query. This is quite common when dumping data into ES just to
investigate it but less common in the traditional time series use case.
But even there it still happens, it is just less impactful. Consider
the default query produced by Kibana's Discover app: a range of 15
minutes and a interval of 30 seconds. This optimization saves something
like 3 to 12 nanoseconds per document, so that 15 minutes would have to
have hundreds of millions of documents for it to be impactful.

Anyway, this commit takes the query into account when precalculating the
buckets. Mostly this is good when you have "dirty data". Immagine
loading 80 billion docs in an index to investigate them. Most of them
have dates around 2015 and 2016 but some have dates in 1970 and
others have dates in 2030. These outlier dates are "dirty" "garbage".
Well, without this change a `date_histogram` across many of these docs
is significantly slowed down because we don't precalculate the range due
to the outliers. That's just rude! So this change takes the query into
account.

The bulk of the code change here is plumbing the query into place. It
turns out that its a *ton* of plumbing, so instead of just adding a
`Query` member in hundreds of args replace `QueryShardContext` with a
new `AggregationContext` which does two things:
1. Has the top level `Query`.
2. Exposes just the parts of `QueryShardContext` that we actually need
   to run aggregation. This lets us simplify a few tests now and will
   let us simplify many, many tests later.