simplified version of TLS patch

This PR cleans up [PR 1763](https://github.com/playframework/playframework/pull/1763) and [PR1809](https://github.com/playframework/playframework/pull/1809) 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.

Please do merge this soon, as keeping track of a the `master` branche's evolution is not the most productive use of my time.