Allow listener to specify an optional request for TLS client auth

https://issues.apache.org/jira/browse/DISPATCH-1635

Dispatch Router currently allows the user to configure a mandatory requirement that TLS client authentication must be used for connections to a TLS port.
For some use-cases it is desirable for some clients to use TLS client-auth and some clients to authenticate via other means. In Java parlance this mode of operation is describing as Wanting TLS client auth rather than Needing.
It would be convenient if the configuration of TLS client auth in Dispatch Router permitted the Want semantics.