Add new config options: enabled + recording

To align with the other agents and to provide greater flexibility, we're introducing two new config options: `enabled` and `recording`, while deprecating `active`.

Here's the expected behavior as described in https://github.com/elastic/apm/issues/92#issuecomment-519752096:

- `enabled` - Should behave identically to `active`, which in turn should be deprecated (this is essentially a rename to get a more meaningful name)
- `recording` - Should default to `true`. Setting to `false` will completely disable the recording and sending of data (transactions, spans, errors, and metrics). It must be possible to toggle `recording` dynamically, either locally or via remote config. Setting `recording` to `false` will therefore not disable remote config polling, config file watching, etc.