We can go one level down in abstraction and avoid using the public
`ReadableStream` APIs.
This patch ~5% perf boost on small ReadableStream:
```
Running 10s test @ http://localhost:8080/
2 threads and 10 connections
Thread Stats Avg Stdev Max +/- Stdev
Latency 148.32us 108.95us 3.88ms 95.71%
Req/Sec 33.24k 2.68k 37.94k 73.76%
668188 requests in 10.10s, 77.74MB read
Requests/sec: 66162.91
Transfer/sec: 7.70MB
```
main:
```
Running 10s test @ http://localhost:8080/
2 threads and 10 connections
Thread Stats Avg Stdev Max +/- Stdev
Latency 150.23us 67.61us 4.39ms 94.80%
Req/Sec 31.81k 1.55k 35.56k 83.17%
639078 requests in 10.10s, 74.36MB read
Requests/sec: 63273.72
Transfer/sec: 7.36MB
```
Fixes: #20569 by introducing a custom replacement for the tokio mpsc
channel that is byte-size backpressure-aware.
Using the testcase in the linked bug, we see all the small writes
aggregated into a single packet and HTTP frame.
```
10:39 $ nc localhost 8000
GET / HTTP/1.1
HTTP/1.1 200 OK
content-type: text/plain
vary: Accept-Encoding
transfer-encoding: chunked
date: Tue, 19 Sep 2023 16:39:13 GMT
A
0
1
2
3
4
```
This patch:
```
Running 10s test @ http://localhost:8080/
2 threads and 10 connections
Thread Stats Avg Stdev Max +/- Stdev
Latency 157.47us 194.89us 9.53ms 98.97%
Req/Sec 31.37k 1.56k 34.73k 85.15%
630407 requests in 10.10s, 73.35MB read
Requests/sec: 62428.12
Transfer/sec: 7.26MB
```
main:
```
Running 10s test @ http://localhost:8080/
2 threads and 10 connections
Thread Stats Avg Stdev Max +/- Stdev
Latency 343.75us 200.48us 10.41ms 98.25%
Req/Sec 14.64k 806.52 16.98k 84.65%
294018 requests in 10.10s, 39.82MB read
Requests/sec: 29109.91
Transfer/sec: 3.94MB
```
---------
Co-authored-by: Bert Belder <bertbelder@gmail.com>
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---------
Co-authored-by: Matt Mastracci <matthew@mastracci.com>
