Allow config options to be set/changed after agent has started

Most config options today could technically easily be updated after the agent has started but there's currently no public API to do so. And some might be impossible to change after start as their values are used during startup - e.g. `serverUrl`, as this is used when initializing the [elastic-apm-http-client](https://github.com/elastic/apm-nodejs-http-client), which is currently only done during start.

Making it possible to change these config options after start will - amongst other things - make it possible to use the Node.js agent to instrument Kibana (as the central Kibana config where you'd normally store the agent config is currently not loaded until after the agent has started).