pkg/fuzzer: pregenerate gen/fuzz requests

[dvyukov]

Mutation can be quite slow and it's on the critical ExchangeInfo path.
Pregenerate gen/fuzz requests in a separate goroutine,
so that they are readily available for consumption most of the time.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.0%. Comparing base (eaa9ee9) to head (359d28c).
Report is 1 commits behind head on master.

Additional details and impacted files

Files	Coverage Δ
pkg/fuzzer/fuzzer.go	82.2% <100.0%> (+1.3%)	⬆️

... and 1 file with indirect coverage changes

[dvyukov]

It looks like a single goroutine can't keep up pregenerating inputs:

I think we need to start NumCPU goroutines + pre-serialize programs in Requests, so that Exchange can just take the byte slice and send.

[a-nogikh]

Did you observe any performance improvements?

[a-nogikh]

And you tested it with the non-instrumented kernel on VMs, right? I think when a kernel is instrumented, and especially when we're on Cloud and have extra network-related delays, the CPU time spent on gen/fuzz operations won't be that much of a bottleneck.