This changes the behavior of aggregations when search is performed
against enough shards to enable "batch reduce" mode. In this case we
force always store aggregations in serialized form rather than a
traditional java reference. This should shrink the memory usage of large
aggregations at the cost of slightly slowing down aggregations where the
coordinating node is also a data node. Because we're only doing this
when there are many shards this is likely to be fairly rare.
As a side effect this lets us add logs for the memory usage of the aggs
buffer:
```
[2020-04-03T17:03:57,052][TRACE][o.e.a.s.SearchPhaseController] [runTask-0] aggs partial reduction [1320->448] max [1320]
[2020-04-03T17:03:57,089][TRACE][o.e.a.s.SearchPhaseController] [runTask-0] aggs partial reduction [1328->448] max [1328]
[2020-04-03T17:03:57,102][TRACE][o.e.a.s.SearchPhaseController] [runTask-0] aggs partial reduction [1328->448] max [1328]
[2020-04-03T17:03:57,103][TRACE][o.e.a.s.SearchPhaseController] [runTask-0] aggs partial reduction [1328->448] max [1328]
[2020-04-03T17:03:57,105][TRACE][o.e.a.s.SearchPhaseController] [runTask-0] aggs final reduction [888] max [1328]
```
These are useful, but you need to keep some things in mind before
trusting them:
1. The buffers are oversized ala Lucene's ArrayUtils. This means that we
are using more space than we need, but probably not much more.
2. Before they are merged the aggregations are inflated into their
traditional Java objects which *probably* take up a lot more space
than the serialized form. That is, after all, the reason why we store
them in serialized form in the first place.
And, just because I can, here is another example of the log:
```
[2020-04-03T17:06:18,731][TRACE][o.e.a.s.SearchPhaseController] [runTask-0] aggs partial reduction [147528->49176] max [147528]
[2020-04-03T17:06:18,750][TRACE][o.e.a.s.SearchPhaseController] [runTask-0] aggs partial reduction [147528->49176] max [147528]
[2020-04-03T17:06:18,809][TRACE][o.e.a.s.SearchPhaseController] [runTask-0] aggs partial reduction [147528->49176] max [147528]
[2020-04-03T17:06:18,827][TRACE][o.e.a.s.SearchPhaseController] [runTask-0] aggs partial reduction [147528->49176] max [147528]
[2020-04-03T17:06:18,829][TRACE][o.e.a.s.SearchPhaseController] [runTask-0] aggs final reduction [98352] max [147528]
```
I got that last one by building a ten shard index with a million docs in
it and running a `sum` in three layers of `terms` aggregations, all on
`long` fields, and with a `batched_reduce_size` of `3`.
62d6bc31bf