Improved Security::PeerConnector::serverConn management

Improved Security::PeerConnector::serverConn management

Similar to the previous branch commit

When sending a negative answer, we would set answer().conn to an open
connection, async-send the answer, and then hurry to close the
connection using our pointer to the shared Connection object. If
everything went according to the plan, the recipient would get a non-nil
but closed Connection object. Now, a negative answer simply means no
connection at all. Same for a tunneled answer.

Probably fixed a bug in the official code where
PeerConnector::negotiate() assumed that a sslFinalized() does not return
true after callBack(). It may (e.g., when CertValidationHelper::Submit()
throws). Same for PeekingPeerConnector::checkForPeekAndSpliceMatched().

Also reduced Security::PeerConnector exposure to a closed Connection
object when serverConn is closed externally.