cleaned up TLS client certificate request support 

This PR cleans up [PR 1763](https://github.com/playframework/playframework/pull/1763) as requested, fulfilling all requirements of #1760 . 

* TLS Client support - a server can request asynchronously client certificates
* WS improvement so that one can have multiple WS objects, for example so that different tests can have different certificates, or that a server can authenticate to different services with different identities.