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---

Many RPC methods require JSON marshalled responses. We saw this taking a
notable amount of heap allocation in query serving full nodes. This PR
removes some extra heap allocations that were being done. We avoided
using the more efficient encoder.Encode before, because it added a
newline. This PR changes the function signature for these private
methods to be using *bytes.Buffer, and then uses the in-buffer methods
(rather than a second copy). We then just truncate the final byte after
each such call, which does not waste any allocations.

I added a benchmark for the most complex test case.

OLD:
```
BenchmarkJsonMarshalStruct-12              78992             15542 ns/op            4487 B/op        191 allocs/op
```
New:
```
BenchmarkJsonMarshalStruct-12              93346             11132 ns/op            3245 B/op         58 allocs/op
```

Roughly a 3-4x reduction in the number of allocations, and 20% speedup.

#### PR checklist

- [x] Tests written/updated - Existing tests cover this
- [x] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)
- [x] Updated relevant documentation (`docs/` or `spec/`) and code
comments
- [x] Title follows the [Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/) spec

(cherry picked from commit eb09601)