Use the wasi functions for time retrieval, and yield less often to the JS engine

[tomaka]

Fix #418

[tomaka]

Everything times out for some reason, and I don't understand why.

[tomaka]

I think that it's because it's become so slow that the CPU is completely clogged.

[tomaka]

The issue happens in cpu_rate_limiter.rs. Switching back to the old Instant in only that module fixes the issue.

I'm still completely missing what could be happening.

[tomaka]

My hypothesis is that the change in precision in the time causes the CPU limiter to call setImmediate very very often, thus preventing any other JS task (such as the connection succeeding) from being executed.

[tomaka]

Confirmed that this was the issue.
We now call setTimeout and setImmediate only after we have accumulated 5ms of lack of sleep.
This solves the problem.