JsonHelper usage in all NGSIv2 renders (second step: optimization and performance)

[fgalan]

JsonHelper class was introduced in PR #1284 as a way of simplifying the JSON response rendering (at that time only used in the JSON response for the "get all entities" operations). In addition, the class allows to rendering logic in a common place, which is interesting as it allows to solve problems such the one described in #1172 in a single point.

This issue is about extending the usage of JsonHelper to the other NGSIv2 types (and adapt the JsonHelper class in the process, if needed). As a result, we should see a reduction of the length and complexity of the current render() methods, as the start to use addRaw(), addString().

Effort: 5 man day

[kzangeli]

Before making this big change, that definitely has its advantages, we need to make sure the broker doesn't loose too much in terms of throughput.

[fgalan]

Actually, what we have to check is not only the change in throughput, but the trade off between throughput and code simplification (i.e. small loss in throughput is acceptable if the gain in code simplification is big).

[kzangeli]

Yes, that is exactly what I said :-)