Investigate improvement in JSON huge payload

[sebastienros]

[BrennanConroy]

Narrowed this down to dotnet/runtime@8f2f3da...010a744
Which is the Json change that shouldn't really affect too much yet 😆

[sebastienros]

But it's also regression workingset/allocations, is that expected? #1994

[BrennanConroy]

Haven't looked at that one yet.

[BrennanConroy]

Allocations are all from PooledByteBufferWriter. I think we basically doubled the size of it by adding a Stream field to it, and making it inherit from PipeWriter which has a private Stream field as well. So it went from int and byte[], to int, byte[], Stream, Stream.