Hand craft datapoint JSON in server responses.

We know the format of what we're marshalling, so can be more efficient than
the generic JSON package.

Benchmark results on 1000 values:
BenchmarkJSONPackageEncoding	   10000	    133631 ns/op
BenchmarkJSONArtisinalEncoding	   50000	     45637 ns/op

In actual usage, drops server response time by ~20%.