perf: improve body read performance

[usualoma]

This PR applies fixes to maintain compatibility with web standards for the request body optimization merged in #301.

It seems we can implement this without significantly reducing the performance gains, but it will likely require a considerable amount of code. Maintenance costs also look substantial.

============================================================
BENCHMARK RESULTS
============================================================

| Benchmark         | npm            | dev            | Difference  |
| ----------------- | -------------- | -------------- | ----------- |
| Average           | 50,051.93      | 59,663.18      | +19.20%     |
| Ping (GET /)      | 64,977.69      | 69,014.04      | +6.21%      |
| Query (GET /id)   | 63,710.19      | 66,806.70      | +4.86%      |
| Body (POST /json) | 21,467.90      | 43,168.79      | +101.09%    |