[performance] switch cache implementation to fastjson

[martinlippert]

At the moment, the IndexCacheOnDisc implementation uses Gson to convert objects to and from JSON. Especially the write operations are executed quite frequently (basically every time a file changes and the cache is updated), switching the JSON conversion part over to fastjson will improve performance.

Side note: with this switch, we would also need to update the custom type adapters that we implemented for Gson to the corresponding mechanism in fastjson.

[martinlippert]

Additional cache-related performance improvements are captured in #1071

[martinlippert]

Due to #1071 being implemented now, the impact of improving the pure speed of the JSON serialization of the symbols/diagnostics cache is not big enough to prioritize this issue at the moment. Closing as declined.