This has the potential to cut the time that players spend at the
"logging in..." (or "encrypting..." for 1.13+) screen by a fair
amount (gains of 200+ ms were noted for my own home connection).
While this sounds minor, I really do like to aim for all the details
and this is one of them.
- Potentially fixed a reference count leak with plugin messages.
- Cleaned up plugin message handling.
- Optimized the pipeline for better throughput by eliminating copying
in the varint encoder and reduced object churn elsewhere.
