[GossipSub 1.1] Limit the size of PRUNE Peer Exchange list

[Nashatyrev]

At the moment the ControlPrune.peers list is not limited by any boundary.

This PR suggests to add the MaxExchangePeers parameter for that.
Instead of hardcoding a 'reasonable default' I'd suggest to parametrize it mostly to have the ability to turn off this feature (by setting this parameter to 0).

See also this issue: ethereum/consensus-specs#3500

[vyzo]

Sounds good to me.

[diegomrsantos]

If to disable the feature we need to set the parameter to 0, making 16 the default value will make it enabled by default.

[diegomrsantos]

@Nashatyrev how about making 0 the default value and effectively disabling it in the implementations? Library users who wish to enable the feature should change it to a number greater than 0.

[Nashatyrev]

@Nashatyrev how about making 0 the default value and effectively disabling it in the implementations? Library users who wish to enable the feature should change it to a number greater than 0.

I don't know actually. There was a reason why this feature was added I believe. And it seems a bit weird having a feature in the spec which is disabled by default.

[diegomrsantos]

It seems that was the intention as mentioned here #570 (comment)