Replace Jackson with custom JSON emitter

[vy]

While Jackson is the de facto JSON serialization library in the Java world and log4j2-logstash-layout enjoys its great performance, flexibility, and customization features, it is currently the slowest chain in the entire serialization pipeline. Given log4j2-logstash-layout only serializes LogEvents into JSON, a custom JSON emitter can very well be implemented in a pretty straightforward way.

Pros

• No external (required) dependency
• Faster serialization

Cons

• A byte[] recycler will need to be implemented to avoid GC
• Jackson is still needed as an optional dependency while serializing Messages that are not SimpleMessage
• null entry elimination might need to be dropped (difficult to get right)

@mkedwards Would you (or your company) be interested in sponsoring such an effort?