Refactor to match golang standard practices

[louisroyer]

• avoid having multiple api packages
• create errors types
• avoid starting a new gorouting each time we get a new packet (

  SRv6/internal/netfunc/netfunc.go

  Line 44 in 699d839

  go func(ctx context.Context, iface *iproute2.TunIface) {

  ): at high rates, this will just consume a lot of memory; use a finite number of workers, and recycle memory using a sync.Pool
• rework the pattern "register task, then run init, then run exit": this creates a lot of boilerplate; when we do Setup.Run(), we should instead start tasks directly, and optionally defer a Close() method