Allow connection reuse for request without credentials when TLS client auth is not in use

[annevk]

The connection separation we have today is the result of TLS client auth which is a property of the connection, rather than the request.

The argument has been made that we should simply tag connections with respect to whether TLS client auth is used. If it is, a request without credentials cannot use that connection.

If it is not, that connection should be up for reuse by both requests with credentials and without.

We might have to cater for connections being made as a result of a request without credentials. If you later do a request with credentials, it might not be able to reuse that connection since that would prevent TLS client auth? (Do we know whether the server tried to use TLS client auth even if the client doesn't want it, then we might be able to optimize this even more.)