Drop pending connections in case of backend down

[hamadodene]

Drop pending connections when backend becomes unreachable to avoid accumulation of unused connections when backend comes back UP.

[NiccoMlt]

I think that the problem here is that we never flush the forwarders cached in the pool in ProxyRequestsManager#reloadConfiguration:99.

These clients are built in the forward method from the connection pool, but it's never cleared, nor adapted when configuration changes

[hamadodene]

Make sense...i think we need to make a Fix