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
```
This commit improves async op sanitizer speed by only delaying metrics
collection if there are pending ops. This
results in a speedup of around 30% for small CPU bound unit tests.
It performs this check and possible delay on every collection now,
fixing an issue with parent test leaks into steps.
