reprovider: add config option to set reprovide interval

[whyrusleeping]

License: MIT
Signed-off-by: Jeromy why@ipfs.io

[Kubuxu]

LGTM

[jbenet]

• i wonder if this will have serious adverse effects by triggering or not triggering a whole large codepath.
• I think this may be more resistant to error and code modification if we always do go n.Reprovider.ProvideEvery(ctx, interval) instead of guarding it in an if clause. The func can treats 0 (or a negative number) as "oh, do nothing? ok and it either waits or exits". Your code becomes:

interval := kReprovideFrequency
if cfg.Reprovider.Interval != "" {
  dur, err := time.ParseDuration(cfg.Reprovider.Interval)
  if err != nil {
    return err
  }
  interval = dur
}
go n.Reprovider.ProvideEvery(ctx, interval)

And you always guard in n.Reprovider.ProvideEvery which should happen already.