These methods are returning nsCString wrappers for static char arrays;
we only need to return nsDependentCStrings, rather than allocating and
copying entirely new nsCString objects.
To cut down on complexity, we don't require specifying any expiry versions.
Given that these events will be recorded non-persistently from off-train add-ons, they can be expired by shipping new add-on releases.
We also start to use the new "record on release" terminology here instead of opt-in/opt-out, but are not changing the internal functionality yet.
Technically, this is implemented by keeping a separate registry for the dynamic event information.
Built-in & dynamic events are tracked with separate numeric ids, so introduce a common identifier for both, an EventKey.
For actual event storage, the events are treated the same as built-in events. They are simply bucketed into the 'dynamic' process storage.
This approach ends up duplicating code paths that use the event info, but keeps a single implementation for recording, storage & serialization.
