Set default value for proxy.config.ssl.number.threads to "-1", to default ET_NET threads to handle SSL connections

Using ET_SSL threads to handle SSL connections affects sharing origin sessions per thread pool (refer TS-2574). This limitation is addressed in TS-2574 which forces to use ET_NET threads for SSL connections, when proxy.config.ssl.number.threads is configured to -1. This bug is to change the current default value "0" for proxy.config.ssl.number.threads to "-1" to make this the default behavior. A separate bug for release 7.0 will be opened to phase out proxy.config.ssl.number.threads completely and always use ET_NET threads for ssl connections