Disable payload compression for localhost

Per https://github.com/elastic/apm/pull/518 agents "SHOULD" disable compression on intake requests to localhost. This includes the lambda environment.

- We should *measure* for the Node.js APM agent, for lambda and not. In a heavy load case this would mean a lot more separate intake requests as `apiRequestBytes` is hit. However, the case in a Lambda will likely differ as a single invocation is always flushed it likely (?) won't hit that apiRequestBytes size.
- The spec PR above suggests for the Lambda ext: "If the agent sends uncompressed data, compress it when reading the data and buffer it in an compressed byte array." Is the extension actually doing this now? My limited read of the `ioutil.ReadAll()` suggests not. So there would presumably be larger (approaching 768kB) blips in memory usage of the ext.