Add rate-limit for outgoing connections

#### Version information:
ipfs version 0.4.19

#### Type:

One of
- "feature": If you'd like to suggest a feature.
- "enhancement ": If you'd like to suggest an improvement on to existing feature.

#### Description:

This is distinct from #2489 and #3320

ipfs is opening outgoing connections at such a rate that it repeatedly triggered the network scan detector of my hoster.
The cheap workaround for now uses `iptables -m limit --limit 12/min -p tcp --sport 4001 --syn ...`
It would be much nicer if the ipfs daemon could be configured to limit itself.