NewRelicHandler: Prevent newrelic_add_custom_parameter() overwriting

Add prefixes to the context and extra record keys. Needed as some processors have the same keys as the the context. Specifically, the `IntrospectionProcessor` has "line" and "file" that can overwrite an error context "line" and "file". If there is something like an E_PARSE error, the context variables are set, but the `IntrospectionProcessor` gets nulls. Tested on Lite account (screenshot below)

![newrelic](https://cloud.githubusercontent.com/assets/1920405/4444811/b9921802-47f3-11e4-88d7-9421ba7c7e35.jpg)