Adds a `parallel` flag to `deno serve`. When present, we spawn multiple
workers to parallelize serving requests.
```bash
deno serve --parallel main.ts
```
Currently on linux we use `SO_REUSEPORT` and rely on the fact that the
kernel will distribute connections in a round-robin manner.
On mac and windows, we sort of emulate this by cloning the underlying
file descriptor and passing a handle to each worker. The connections
will not be guaranteed to be fairly distributed (and in practice almost
certainly won't be), but the distribution is still spread enough to
provide a significant performance increase.
---
(Run on an Macbook Pro with an M3 Max, serving `deno.com`
baseline::
```
❯ wrk -d 30s -c 125 --latency http://127.0.0.1:8000
Running 30s test @ http://127.0.0.1:8000
2 threads and 125 connections
Thread Stats Avg Stdev Max +/- Stdev
Latency 239.78ms 13.56ms 330.54ms 79.12%
Req/Sec 258.58 35.56 360.00 70.64%
Latency Distribution
50% 236.72ms
75% 248.46ms
90% 256.84ms
99% 268.23ms
15458 requests in 30.02s, 2.47GB read
Requests/sec: 514.89
Transfer/sec: 84.33MB
```
this PR (`with --parallel` flag)
```
❯ wrk -d 30s -c 125 --latency http://127.0.0.1:8000
Running 30s test @ http://127.0.0.1:8000
2 threads and 125 connections
Thread Stats Avg Stdev Max +/- Stdev
Latency 117.40ms 142.84ms 590.45ms 79.07%
Req/Sec 1.33k 175.19 1.77k 69.00%
Latency Distribution
50% 22.34ms
75% 223.67ms
90% 357.32ms
99% 460.50ms
79636 requests in 30.07s, 12.74GB read
Requests/sec: 2647.96
Transfer/sec: 433.71MB
```
