Maximum precision for float serialization

We should explicitly set the number of decimal places to use for float serialization. This will probably have to be done by subclassing the default `JSONEncoder` and using a custom float wrapper that knows how to serialize itself with a certain precision. It might also be possible using the standard library `Decimal` type. We should make sure to consider the impact on the performance of serialization.

https://docs.python.org/3.9/library/json.html#json.JSONEncoder

Fields affected:
- `queue_pop_time_delta` in `matchmaker_info`