Consider a lower level/more performant response representation

[milessabin]

Currently we construct responses as Circe JsonObjects. @djspiewak observed that this is very costly, and also most likely unnecessary given that we're constructing a single response value bottom up: we could just as easily glom byte arrays together rather than constructing an intermediate structured value.

Simply ripping out Circe (for this role, not everywhere) and replacing with something lower level should be fairly straightforward, but probably isn't an immediate priority.

A slightly more ambitious move would be to support response streaming (ie. start returning the response to the client as soon as the first bytes are available). This might involve some tricky scheduling of nested subqueries for the best results, but I think also should be possible.

[tpolecat]

Not super concerned about this right now, but streaming seems like a really interesting problem.