Add support for "redacted keys"

[imjoehaines]

Goal

This PR adds a new redacted_keys configuration option, which will replace meta_data_filters in the next major version

As well as being consistent with other notifier APIs, redacted_keys uses a case-insensitive, exact comparison. For example, the string abc will match abc and ABC but not abcd

This makes string matching more useful as it's less likely to hit a false positive, but the old behaviour is still possible by using a Regexp