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

• Tests written/updated - Existing tests cover this
• Changelog entry added in .changelog (we use unclog to manage our changelog)
• Updated relevant documentation (docs/ or spec/) and code comments
• Title follows the Conventional Commits spec