Fixes#8079
Because WiFiClientSecure inherits WiFiClient, and WiFiClientSecureCtx also
inherits WiFiClient, they both end up in the list of TCP connections that
are used for WiFiClient::stopAllExcept(). This would cause the underlying
SSL connection to be closed whenever you attempted to
stopAllExcept(WiFiClientSecure)
Fix by adding a "_owned"(by) pointer in the WiFiClient object which points to
nullptr (default case) or to the associated lower-layer connection.
When stopping all connections except one, only look at the lowermost
connections.
45e7976c50