Support to remove peers from peer client poll as they exit

[asifsmohammed]

Is your feature request related to a problem? Please describe. Core peer forwarder currently stores all the peers in a map as they are made available but it does not remove from the map as they exit.

Describe the solution you'd like Clean up the resources in shutdown call of PeerForwardingProcessorDecorator.

Additional context This is the same behavior even in the peer forwarder plugin here.

There is an existing TODO in peer forwarder plugin to do it.

[dlvenable]

We can handle this after releasing 2.0.